# N-queens problem

(Redirected from N-Queens)
You are encouraged to solve this task according to the task description, using any language you may know.

Solve the eight queens puzzle.

You can extend the problem to solve the puzzle with a board of size   NxN.

For the number of solutions for small values of   N,   see   OEIS: A000170.

## 11l

Translation of: Nim
```-V BoardSize = 8

F underAttack(col, queens)
I col C queens
R 1B
L(x) queens
I abs(col - x) == queens.len - L.index
R 1B
R 0B

F solve(n)
V result = [[Int]()]
[[Int]] newSolutions
L(row) 1 .. n
L(solution) result
L(i) 1 .. BoardSize
I !underAttack(i, solution)
newSolutions.append(solution [+] [i])
swap(&result, &newSolutions)
newSolutions.clear()
R result

print(‘Solutions for a chessboard of size ’String(BoardSize)‘x’String(BoardSize))
print()

V row = L.index
I row > 0
print(‘ ’, end' ‘’)
print(Char(code' ‘a’.code + row)‘’col, end' ‘’)
print(end' I L.index % 4 == 3 {"\n"} E ‘      ’)```
Output:
```Solutions for a chessboard of size 8x8

a1 b5 c8 d6 e3 f7 g2 h4      a1 b6 c8 d3 e7 f4 g2 h5      a1 b7 c4 d6 e8 f2 g5 h3      a1 b7 c5 d8 e2 f4 g6 h3
a2 b4 c6 d8 e3 f1 g7 h5      a2 b5 c7 d1 e3 f8 g6 h4      a2 b5 c7 d4 e1 f8 g6 h3      a2 b6 c1 d7 e4 f8 g3 h5
a2 b6 c8 d3 e1 f4 g7 h5      a2 b7 c3 d6 e8 f5 g1 h4      a2 b7 c5 d8 e1 f4 g6 h3      a2 b8 c6 d1 e3 f5 g7 h4
a3 b1 c7 d5 e8 f2 g4 h6      a3 b5 c2 d8 e1 f7 g4 h6      a3 b5 c2 d8 e6 f4 g7 h1      a3 b5 c7 d1 e4 f2 g8 h6
a3 b5 c8 d4 e1 f7 g2 h6      a3 b6 c2 d5 e8 f1 g7 h4      a3 b6 c2 d7 e1 f4 g8 h5      a3 b6 c2 d7 e5 f1 g8 h4
a3 b6 c4 d1 e8 f5 g7 h2      a3 b6 c4 d2 e8 f5 g7 h1      a3 b6 c8 d1 e4 f7 g5 h2      a3 b6 c8 d1 e5 f7 g2 h4
a3 b6 c8 d2 e4 f1 g7 h5      a3 b7 c2 d8 e5 f1 g4 h6      a3 b7 c2 d8 e6 f4 g1 h5      a3 b8 c4 d7 e1 f6 g2 h5
a4 b1 c5 d8 e2 f7 g3 h6      a4 b1 c5 d8 e6 f3 g7 h2      a4 b2 c5 d8 e6 f1 g3 h7      a4 b2 c7 d3 e6 f8 g1 h5
a4 b2 c7 d3 e6 f8 g5 h1      a4 b2 c7 d5 e1 f8 g6 h3      a4 b2 c8 d5 e7 f1 g3 h6      a4 b2 c8 d6 e1 f3 g5 h7
a4 b6 c1 d5 e2 f8 g3 h7      a4 b6 c8 d2 e7 f1 g3 h5      a4 b6 c8 d3 e1 f7 g5 h2      a4 b7 c1 d8 e5 f2 g6 h3
a4 b7 c3 d8 e2 f5 g1 h6      a4 b7 c5 d2 e6 f1 g3 h8      a4 b7 c5 d3 e1 f6 g8 h2      a4 b8 c1 d3 e6 f2 g7 h5
a4 b8 c1 d5 e7 f2 g6 h3      a4 b8 c5 d3 e1 f7 g2 h6      a5 b1 c4 d6 e8 f2 g7 h3      a5 b1 c8 d4 e2 f7 g3 h6
a5 b1 c8 d6 e3 f7 g2 h4      a5 b2 c4 d6 e8 f3 g1 h7      a5 b2 c4 d7 e3 f8 g6 h1      a5 b2 c6 d1 e7 f4 g8 h3
a5 b2 c8 d1 e4 f7 g3 h6      a5 b3 c1 d6 e8 f2 g4 h7      a5 b3 c1 d7 e2 f8 g6 h4      a5 b3 c8 d4 e7 f1 g6 h2
a5 b7 c1 d3 e8 f6 g4 h2      a5 b7 c1 d4 e2 f8 g6 h3      a5 b7 c2 d4 e8 f1 g3 h6      a5 b7 c2 d6 e3 f1 g4 h8
a5 b7 c2 d6 e3 f1 g8 h4      a5 b7 c4 d1 e3 f8 g6 h2      a5 b8 c4 d1 e3 f6 g2 h7      a5 b8 c4 d1 e7 f2 g6 h3
a6 b1 c5 d2 e8 f3 g7 h4      a6 b2 c7 d1 e3 f5 g8 h4      a6 b2 c7 d1 e4 f8 g5 h3      a6 b3 c1 d7 e5 f8 g2 h4
a6 b3 c1 d8 e4 f2 g7 h5      a6 b3 c1 d8 e5 f2 g4 h7      a6 b3 c5 d7 e1 f4 g2 h8      a6 b3 c5 d8 e1 f4 g2 h7
a6 b3 c7 d2 e4 f8 g1 h5      a6 b3 c7 d2 e8 f5 g1 h4      a6 b3 c7 d4 e1 f8 g2 h5      a6 b4 c1 d5 e8 f2 g7 h3
a6 b4 c2 d8 e5 f7 g1 h3      a6 b4 c7 d1 e3 f5 g2 h8      a6 b4 c7 d1 e8 f2 g5 h3      a6 b8 c2 d4 e1 f7 g5 h3
a7 b1 c3 d8 e6 f4 g2 h5      a7 b2 c4 d1 e8 f5 g3 h6      a7 b2 c6 d3 e1 f4 g8 h5      a7 b3 c1 d6 e8 f5 g2 h4
a7 b3 c8 d2 e5 f1 g6 h4      a7 b4 c2 d5 e8 f1 g3 h6      a7 b4 c2 d8 e6 f1 g3 h5      a7 b5 c3 d1 e6 f8 g2 h4
a8 b2 c4 d1 e7 f5 g3 h6      a8 b2 c5 d3 e1 f7 g4 h6      a8 b3 c1 d6 e2 f5 g7 h4      a8 b4 c1 d3 e6 f2 g7 h5
```

## 360 Assembly

Translation of: FORTRAN

Translated from the Fortran 77 solution.
For maximum compatibility, this program uses only the basic instruction set (S/360).

```*        N-QUEENS PROBLEM          04/09/2015
MACRO
&LAB     XDECO  &REG,&TARGET
&LAB     B      I&SYSNDX           branch around work area
P&SYSNDX DS     0D,PL8             packed
W&SYSNDX DS     CL13               char
I&SYSNDX CVD    &REG,P&SYSNDX          convert to decimal
EDMK   W&SYSNDX,P&SYSNDX+2    edit and mark
BCTR   R1,0                   locate the right place
MVC    0(1,R1),W&SYSNDX+12    move the sign
MVC    &TARGET.(12),W&SYSNDX  move to target
MEND
NQUEENS  CSECT
SAVE   (14,12)            save registers on entry
USING  *,R12              set base register
LA     R11,SAVEA          mySA
LR     R13,R11            set mySA pointer
LA     R7,LL              l
LA     R6,1               i=1
LOOPI    LR     R1,R6              do i=1 to l
SLA    R1,1               i*2
STH    R6,A-2(R1)         a(i)=i
LA     R6,1(R6)           i=i+1
BCT    R7,LOOPI           loop do i
OPENEM   OPEN   (OUTDCB,OUTPUT)    open the printer file
LA     R9,1               n=1 start of loop
LOOPN    CH     R9,L               do n=1 to l
BH     ELOOPN             if n>l then exit loop
SR     R8,R8              m=0
LA     R10,1              i=1
LR     R5,R9              n
SLA    R5,1               n*2
BCTR   R5,0               r=2*n-1
E40      CR     R10,R9             if i>n
BH     E80                then goto e80
LR     R11,R10            j=i
E50      LR     R1,R10             i
SLA    R1,1               i*2
LA     R6,A-2(R1)         r6=@a(i)
LR     R1,R11             j
SLA    R1,1               j*2
LA     R7,A-2(R1)         r7=@a(j)
MVC    Z,0(R6)            z=a(i)
MVC    Y,0(R7)            y=a(j)
LR     R3,R10             i
SH     R3,Y               -y
AR     R3,R9              p=i-y+n
LR     R4,R10             i
AH     R4,Y               +y
BCTR   R4,0               q=i+y-1
MVC    0(2,R6),Y          a(i)=y
MVC    0(2,R7),Z          a(j)=z
LR     R1,R3              p
SLA    R1,1               p*2
LH     R2,U-2(R1)         u(p)
LTR    R2,R2              if u(p)<>0
BNE    E60                then goto e60
LR     R1,R4              q
AR     R1,R5              q+r
SLA    R1,1               (q+r)*2
LH     R2,U-2(R1)         u(q+r)
C      R2,=F'0'           if u(q+r)<>0
BNE    E60                then goto e60
LR     R1,R10             i
SLA    R1,1               i*2
STH    R11,S-2(R1)        s(i)=j
LA     R0,1               r0=1
LR     R1,R3              p
SLA    R1,1               p*2
STH    R0,U-2(R1)         u(p)=1
LR     R1,R4              q
AR     R1,R5              q+r
SLA    R1,1               (q+r)*2
STH    R0,U-2(R1)         u(q+r)=1
LA     R10,1(R10)         i=i+1
B      E40                goto e40
E60      LA     R11,1(R11)         j=j+1
CR     R11,R9             if j<=n
BNH    E50                then goto e50
E70      BCTR   R11,0              j=j-1
CR     R11,R10            if j=i
BE     E90                goto e90
LR     R1,R10             i
SLA    R1,1               i*2
LA     R6,A-2(R1)         r6=@a(i)
LR     R1,R11             j
SLA    R1,1               j*2
LA     R7,A-2(R1)         r7=@a(j)
MVC    Z,0(R6)            z=a(i)
MVC    0(2,R6),0(R7)      a(i)=a(j)
MVC    0(2,R7),Z          a(j)=z;
B      E70                goto e70
E80      LA     R8,1(R8)           m=m+1
E90      BCTR   R10,0              i=i-1
LTR    R10,R10            if i=0
BZ     ZERO               then goto zero
LR     R1,R10             i
SLA    R1,1               i*2
LH     R2,A-2(R1)         r2=a(i)
LR     R3,R10             i
SR     R3,R2              -a(i)
AR     R3,R9              p=i-a(i)+n
LR     R4,R10             i
AR     R4,R2              +a(i)
BCTR   R4,0               q=i+a(i)-1
LR     R1,R10             i
SLA    R1,1               i*2
LH     R11,S-2(R1)        j=s(i)
LA     R0,0               r0=0
LR     R1,R3              p
SLA    R1,1               p*2
STH    R0,U-2(R1)         u(p)=0
LR     R1,R4              q
AR     R1,R5              q+r
SLA    R1,1               (q+r)*2
STH    R0,U-2(R1)         u(q+r)=0
B      E60                goto e60
ZERO     XDECO  R9,PG+0            edit N
XDECO  R8,PG+12           edit M
PUT    OUTDCB,PG          print buffer
LA     R9,1(R9)           n=n+1
B      LOOPN              loop do n
ELOOPN   CLOSE  (OUTDCB)           close output
L      R13,SAVEA+4        previous save area addrs
LTORG
SAVEA    DS     18F                save area for chaining
OUTDCB   DCB    DSORG=PS,MACRF=PM,DDNAME=OUTDD  use OUTDD in jcl
LL       EQU    14                 ll<=16
L        DC     AL2(LL)            input value
A        DS     (LL)H
S        DS     (LL)H
Z        DS     H
Y        DS     H
PG       DS     CL24               buffer
U        DC     (4*LL-2)H'0'       stack
REGS                      make sure to include copybook jcl
END    NQUEENS```
Output:
```           1           1
2           0
3           0
4           2
5          10
6           4
7          40
8          92
9         352
10         724
11        2680
12       14200
13       47600
14      365596
```

## ABAP

```TYPES: BEGIN OF gty_matrix,
1  TYPE c,
2  TYPE c,
3  TYPE c,
4  TYPE c,
5  TYPE c,
6  TYPE c,
7  TYPE c,
8  TYPE c,
9  TYPE c,
10 TYPE c,
END OF gty_matrix,
gty_t_matrix TYPE STANDARD TABLE OF gty_matrix INITIAL SIZE 8.

DATA: gt_matrix TYPE gty_t_matrix,
gs_matrix TYPE gty_matrix,
gv_count  TYPE i VALUE 0,
gv_solut  TYPE i VALUE 0.

SELECTION-SCREEN BEGIN OF BLOCK b01 WITH FRAME TITLE text-b01.
PARAMETERS: p_number TYPE i OBLIGATORY DEFAULT 8.
SELECTION-SCREEN END OF BLOCK b01.

" Filling empty table
START-OF-SELECTION.
DO p_number TIMES.
APPEND gs_matrix TO gt_matrix.
ENDDO.

" Recursive Function
PERFORM fill_matrix USING gv_count 1 1 CHANGING gt_matrix.
BREAK-POINT.
*&---------------------------------------------------------------------*
*&      Form  FILL_MATRIX
*----------------------------------------------------------------------*
FORM fill_matrix  USING    p_count TYPE i
p_i     TYPE i
p_j     TYPE i
CHANGING p_matrix TYPE gty_t_matrix.

DATA: lv_i      TYPE i,
lv_j      TYPE i,
lv_result TYPE c LENGTH 1,
lt_matrix TYPE gty_t_matrix,
lv_count  TYPE i,
lv_value  TYPE c.

lt_matrix[] = p_matrix[].
lv_count = p_count.
lv_i = p_i.
lv_j = p_j.

WHILE lv_i LE p_number.
WHILE lv_j LE p_number.
CLEAR lv_result.
PERFORM check_position USING lv_i lv_j CHANGING lv_result lt_matrix.
IF lv_result NE 'X'.
MOVE 'X' TO lv_value.
PERFORM get_position USING lv_i lv_j 'U' CHANGING lv_value lt_matrix.
IF lv_count EQ p_number.
PERFORM show_matrix USING lt_matrix.
ELSE.
PERFORM fill_matrix USING lv_count lv_i lv_j CHANGING lt_matrix.
ENDIF.
lv_value = space.
PERFORM get_position USING lv_i lv_j 'U' CHANGING lv_value lt_matrix.
SUBTRACT 1 FROM lv_count.
ENDIF.
ENDWHILE.
lv_j = 1.
ENDWHILE.
ENDFORM.                    " FILL_MATRIX

*&---------------------------------------------------------------------*
*&      Form  CHECK_POSITION
*&---------------------------------------------------------------------*
FORM check_position  USING value(p_i)  TYPE i
value(p_j)  TYPE i
CHANGING p_result TYPE c
p_matrix TYPE gty_t_matrix.

PERFORM get_position USING p_i p_j 'R' CHANGING p_result p_matrix.
CHECK p_result NE 'X'.

PERFORM check_horizontal USING p_i p_j CHANGING p_result p_matrix.
CHECK p_result NE 'X'.

PERFORM check_vertical USING p_i p_j CHANGING p_result p_matrix.
CHECK p_result NE 'X'.

PERFORM check_diagonals USING p_i p_j CHANGING p_result p_matrix.

ENDFORM.                    " CHECK_POSITION

*&---------------------------------------------------------------------*
*&      Form  GET_POSITION
*&---------------------------------------------------------------------*
FORM get_position  USING value(p_i)      TYPE i
value(p_j)      TYPE i
value(p_action) TYPE c
CHANGING p_result  TYPE c
p_matrix  TYPE gty_t_matrix.

FIELD-SYMBOLS: <fs_lmatrix> TYPE gty_matrix,
<fs_lfield> TYPE any.

READ TABLE p_matrix ASSIGNING <fs_lmatrix> INDEX p_i.
ASSIGN COMPONENT p_j OF STRUCTURE <fs_lmatrix> TO <fs_lfield>.

CASE p_action.
WHEN 'U'.
<fs_lfield> = p_result.
WHEN 'R'.
p_result = <fs_lfield>.
WHEN OTHERS.
ENDCASE.

ENDFORM.                    " GET_POSITION

*&---------------------------------------------------------------------*
*&      Form  CHECK_HORIZONTAL
*&---------------------------------------------------------------------*
FORM check_horizontal  USING value(p_i)      TYPE i
value(p_j)      TYPE i
CHANGING p_result  TYPE c
p_matrix  TYPE gty_t_matrix.
DATA: lv_j TYPE i,
ls_matrix TYPE gty_matrix.

FIELD-SYMBOLS <fs> TYPE c.

lv_j = 1.
READ TABLE p_matrix INTO ls_matrix INDEX p_i.
WHILE lv_j LE p_number.
ASSIGN COMPONENT lv_j OF STRUCTURE ls_matrix TO <fs>.
IF <fs> EQ 'X'.
p_result = 'X'.
RETURN.
ENDIF.
ENDWHILE.
ENDFORM.                    " CHECK_HORIZONTAL

*&---------------------------------------------------------------------*
*&      Form  CHECK_VERTICAL
*&---------------------------------------------------------------------*
FORM check_vertical  USING value(p_i)      TYPE i
value(p_j)      TYPE i
CHANGING p_result  TYPE c
p_matrix  TYPE gty_t_matrix.
DATA: lv_i TYPE i,
ls_matrix TYPE gty_matrix.

FIELD-SYMBOLS <fs> TYPE c.

lv_i = 1.
WHILE lv_i LE p_number.
READ TABLE p_matrix INTO ls_matrix INDEX lv_i.
ASSIGN COMPONENT p_j OF STRUCTURE ls_matrix TO <fs>.
IF <fs> EQ 'X'.
p_result = 'X'.
RETURN.
ENDIF.
ENDWHILE.
ENDFORM.                    " CHECK_VERTICAL

*&---------------------------------------------------------------------*
*&      Form  CHECK_DIAGONALS
*&---------------------------------------------------------------------*
FORM check_diagonals  USING value(p_i)      TYPE i
value(p_j)      TYPE i
CHANGING p_result  TYPE c
p_matrix  TYPE gty_t_matrix.
DATA: lv_dx TYPE i,
lv_dy TYPE i.

* I++ J++ (Up Right)
lv_dx = 1.
lv_dy = 1.
PERFORM check_diagonal USING p_i p_j lv_dx lv_dy CHANGING p_result p_matrix.
CHECK p_result NE 'X'.

* I-- J-- (Left Down)
lv_dx = -1.
lv_dy = -1.
PERFORM check_diagonal USING p_i p_j lv_dx lv_dy CHANGING p_result p_matrix.
CHECK p_result NE 'X'.

* I++ J-- (Right Down)
lv_dx = 1.
lv_dy = -1.
PERFORM check_diagonal USING p_i p_j lv_dx lv_dy CHANGING p_result p_matrix.
CHECK p_result NE 'X'.

* I-- J++ (Left Up)
lv_dx = -1.
lv_dy = 1.
PERFORM check_diagonal USING p_i p_j lv_dx lv_dy CHANGING p_result p_matrix.
CHECK p_result NE 'X'.
ENDFORM.                    " CHECK_DIAGONALS

*&---------------------------------------------------------------------*
*&      Form  CHECK_DIAGONAL
*&---------------------------------------------------------------------*
FORM check_diagonal  USING value(p_i)      TYPE i
value(p_j)      TYPE i
value(p_dx)      TYPE i
value(p_dy)      TYPE i
CHANGING p_result  TYPE c
p_matrix  TYPE gty_t_matrix.
DATA: lv_i TYPE i,
lv_j TYPE i,
ls_matrix TYPE gty_matrix.

FIELD-SYMBOLS <fs> TYPE c.

lv_i = p_i.
lv_j = p_j.
WHILE 1 EQ 1.
ADD: p_dx TO lv_i, p_dy TO lv_j.

IF p_dx EQ 1.
IF lv_i GT p_number. EXIT. ENDIF.
ELSE.
IF lv_i LT 1. EXIT. ENDIF.
ENDIF.

IF p_dy EQ 1.
IF lv_j GT p_number. EXIT. ENDIF.
ELSE.
IF lv_j LT 1. EXIT. ENDIF.
ENDIF.

READ TABLE p_matrix INTO ls_matrix INDEX lv_i.
ASSIGN COMPONENT lv_j OF STRUCTURE ls_matrix TO <fs>.
IF <fs> EQ 'X'.
p_result = 'X'.
RETURN.
ENDIF.
ENDWHILE.
ENDFORM.                    " CHECK_DIAGONAL
*&---------------------------------------------------------------------*
*&      Form  SHOW_MATRIX
*----------------------------------------------------------------------*
FORM show_matrix USING p_matrix TYPE gty_t_matrix.
DATA: lt_matrix TYPE gty_t_matrix,
lv_j      TYPE i VALUE 1,
lv_colum  TYPE string VALUE '-'.

FIELD-SYMBOLS: <fs_matrix> TYPE gty_matrix,
<fs_field>  TYPE c.

WRITE:/ 'Solution: ', gv_solut.

DO p_number TIMES.
CONCATENATE lv_colum '----' INTO lv_colum.
ENDDO.

LOOP AT p_matrix ASSIGNING <fs_matrix>.
IF sy-tabix EQ 1.
WRITE:/ lv_colum.
ENDIF.
WRITE:/ '|'.
DO p_number TIMES.
ASSIGN COMPONENT lv_j OF STRUCTURE <fs_matrix> TO <fs_field>.
IF <fs_field> EQ space.
WRITE: <fs_field> ,'|'.
ELSE.
WRITE: <fs_field> COLOR 2 HOTSPOT ON,'|'.
ENDIF.
ENDDO.
lv_j = 1.
WRITE: / lv_colum.
ENDLOOP.

SKIP 1.
ENDFORM.                    " SHOW_MATRIX
```

```with Ada.Text_IO;  use Ada.Text_IO;

procedure Queens is
Board : array (1..8, 1..8) of Boolean := (others => (others => False));
function Test (Row, Column : Integer) return Boolean is
begin
for J in 1..Column - 1 loop
if (  Board (Row, J)
or else
(Row > J and then Board (Row - J, Column - J))
or else
(Row + J <= 8 and then Board (Row + J, Column - J))
)  then
return False;
end if;
end loop;
return True;
end Test;
function Fill (Column : Integer) return Boolean is
begin
for Row in Board'Range (1) loop
if Test (Row, Column) then
Board (Row, Column) := True;
if Column = 8 or else Fill (Column + 1) then
return True;
end if;
Board (Row, Column) := False;
end if;
end loop;
return False;
end Fill;
begin
if not Fill (1) then
raise Program_Error;
end if;
for I in Board'Range (1) loop
Put (Integer'Image (9 - I));
for J in Board'Range (2) loop
if Board (I, J) then
Put ("|Q");
elsif (I + J) mod 2 = 1 then
Put ("|/");
else
Put ("| ");
end if;
end loop;
Put_Line ("|");
end loop;
Put_Line ("   A B C D E F G H");
end Queens;
```
Output:
``` 8|Q|/| |/| |/| |/|
7|/| |/| |/| |Q| |
6| |/| |/|Q|/| |/|
5|/| |/| |/| |/|Q|
4| |Q| |/| |/| |/|
3|/| |/|Q|/| |/| |
2| |/| |/| |Q| |/|
1|/| |Q| |/| |/| |
A B C D E F G H
```

### Alternate solution

Translation of: Fortran

This one only counts solutions, though it's easy to do something else with each one (instead of the `M := M + 1;` line).

```with Ada.Text_IO;

procedure CountQueens is
function Queens (N : Integer) return Long_Integer is
A : array (0 .. N) of Integer;
U : array (0 .. 2 * N - 1) of Boolean := (others => true);
V : array (0 .. 2 * N - 1) of Boolean := (others => true);
M : Long_Integer := 0;

procedure Sub (I: Integer) is
K, P, Q: Integer;
begin
if N = I then
M := M + 1;
else
for J in I .. N - 1 loop
P := I + A (J);
Q := I + N - 1 - A (J);
if U (P) and then V (Q) then
U (P) := false;
V (Q) := false;
K := A (I);
A (I) := A (J);
A (J) := K;
Sub (I + 1);
U (P) := true;
V (Q) := true;
K := A (I);
A (I) := A (J);
A (J) := K;
end if;
end loop;
end if;
end Sub;
begin
for I in 0 .. N - 1 loop
A (I) := I;
end loop;
Sub (0);
return M;
end Queens;
begin
for N in 1 .. 16 loop
Put (Integer'Image (N));
Put (" ");
Put_Line (Long_Integer'Image (Queens (N)));
end loop;
end CountQueens;
```

## ALGOL 68

Translation of: C
Works with: ALGOL 68 version Standard - no extensions to language used
Works with: ALGOL 68G version Any - tested with release 1.18.0-9h.tiny
Works with: ELLA ALGOL 68 version Any (with appropriate job cards) - tested with release 1.8.8d.fc9.i386
```INT ofs = 1, # Algol68 normally uses array offset of 1 #
dim = 8; # dim X dim chess board #
[ofs:dim+ofs-1]INT b;

PROC unsafe = (INT y)BOOL:(
INT i, t, x;
x := b[y];
FOR i TO y - LWB b DO
t := b[y - i];
IF t = x THEN break true
ELIF t = x - i THEN break true
ELIF t = x + i THEN break true
FI
OD;
FALSE EXIT
break true:
TRUE
);

INT s := 0;

PROC print board = VOID:(
INT x, y;
print((new line, "Solution # ", s+:=1, new line));
FOR y FROM LWB b TO UPB b DO
FOR x FROM LWB b TO UPB b DO
print("|"+(b[y]=x|"Q"|: ODD(x+y)|"/"|" "))
OD;
print(("|", new line))
OD
);

main: (
INT y := LWB b;
b[LWB b] := LWB b - 1;
FOR i WHILE y >= LWB b DO
WHILE
b[y]+:=1;
# BREAK # IF b[y] <= UPB b THEN unsafe(y) ELSE FALSE FI
DO SKIP OD;
IF b[y] <= UPB b  THEN
IF y < UPB b THEN
b[y+:=1] := LWB b - 1
ELSE
print board
FI
ELSE
y-:=1
FI
OD
)```

## APL

Works with: Dyalog APL

More or less copied from the "DFS" lesson on tryapl.org .

```⍝Solution
accm←{⍺,((⍴⍵)=⍴⊃⍺)↑⊂⍵}
atk←{∪∊(⊂⍵)+¯1 0 1×⊂⌽⍳⍴⍵}
dfs←{⊃∇⍨/⌽(⊂⍺ ⍺⍺ ⍵),⍺ ⍵⍵ ⍵}
qfmt←{⍵∘.=⍳⍴⍵}
subs←{(⊂⍵),¨(⍳⍴⊃⍺)~atk ⍵}
queens←{qfmt¨(↓0 ⍵⍴0)accm dfs subs ⍬}

⍝Example
printqueens 6
```
Output:
``` answer  1
0 1 0 0 0 0
0 0 0 1 0 0
0 0 0 0 0 1
1 0 0 0 0 0
0 0 1 0 0 0
0 0 0 0 1 0
0 0 1 0 0 0
0 0 0 0 0 1
0 1 0 0 0 0
0 0 0 0 1 0
1 0 0 0 0 0
0 0 0 1 0 0
0 0 0 1 0 0
1 0 0 0 0 0
0 0 0 0 1 0
0 1 0 0 0 0
0 0 0 0 0 1
0 0 1 0 0 0
0 0 0 0 1 0
0 0 1 0 0 0
1 0 0 0 0 0
0 0 0 0 0 1
0 0 0 1 0 0
0 1 0 0 0 0
```

## AppleScript

```-- Finds all possible solutions and the unique patterns.

property Grid_Size : 8

property Patterns : {}
property Solutions : {}
property Test_Count : 0

property Rotated : {}

on run
local diff
local endTime
local msg
local rows
local startTime

set Patterns to {}
set Solutions to {}
set Rotated to {}

set Test_Count to 0

set rows to Make_Empty_List(Grid_Size)

set startTime to current date
Solve(1, rows)
set endTime to current date
set diff to endTime - startTime

set msg to ("Found " & (count Solutions) & " solutions with " & (count Patterns) & " patterns in " & diff & " seconds.") as text

return Solutions
end run

on Solve(row as integer, rows as list)
if row is greater than (count rows) then
Append_Solution(rows)
return
end if

repeat with column from 1 to Grid_Size
set Test_Count to Test_Count + 1
if Place_Queen(column, row, rows) then
Solve(row + 1, rows)
end if
end repeat
end Solve

on abs(n)
if n < 0 then
-n
else
n
end if
end abs

on Place_Queen(column as integer, row as integer, rows as list)
local colDiff
local previousRow
local rowDiff
local testColumn

repeat with previousRow from 1 to (row - 1)
set testColumn to item previousRow of rows

if testColumn is equal to column then
return false
end if

set colDiff to abs(testColumn - column) as integer
set rowDiff to row - previousRow
if colDiff is equal to rowDiff then
return false
end if
end repeat

set item row of rows to column
return true
end Place_Queen

on Append_Solution(rows as list)
local column
local rowsCopy
local testReflection
local testReflectionText
local testRotation
local testRotationText
local testRotations

copy rows to rowsCopy
set end of Solutions to rowsCopy
local rowsCopy

copy rows to testRotation
set testRotations to {}
repeat 3 times
set testRotation to Rotate(testRotation)
set testRotationText to testRotation as text
if Rotated contains testRotationText then
return
end if
set end of testRotations to testRotationText

set testReflection to Reflect(testRotation)
set testReflectionText to testReflection as text
if Rotated contains testReflectionText then
return
end if
set end of testRotations to testReflectionText
end repeat

repeat with testRotationText in testRotations
set end of Rotated to (contents of testRotationText)
end repeat
set end of Rotated to (rowsCopy as text)
set end of Rotated to (Reflect(rowsCopy) as text)

set end of Patterns to rowsCopy
end Append_Solution

on Make_Empty_List(depth as integer)
local i
local emptyList

set emptyList to {}
repeat with i from 1 to depth
set end of emptyList to missing value
end repeat
return emptyList
end Make_Empty_List

on Rotate(rows as list)
local column
local newColumn
local newRow
local newRows
local row
local rowCount

set rowCount to (count rows)
set newRows to Make_Empty_List(rowCount)
repeat with row from 1 to rowCount
set column to (contents of item row of rows)
set newRow to column
set newColumn to rowCount - row + 1
set item newRow of newRows to newColumn
end repeat

return newRows
end Rotate

on Reflect(rows as list)
local column
local newRows

set newRows to {}
repeat with column in rows
set end of newRows to (count rows) - column + 1
end repeat

return newRows
end Reflect
```

## Arc

This program prints out all possible solutions:

```(def nqueens (n (o queens))
(if (< len.queens n)
(let row (if queens (+ 1 queens.0.0) 0)
(each col (range 0 (- n 1))
(let new-queens (cons (list row col) queens)
(if (no conflicts.new-queens)
(nqueens n new-queens)))))
(prn queens)))

; check if the first queen in 'queens' lies on the same column or diagonal as
; any of the others
(def conflicts (queens)
(let (curr . rest) queens
(or (let curr-column curr.1
(some curr-column (map [_ 1] rest)))  ; columns
(some [diagonal-match curr _] rest))))

(def diagonal-match (curr other)
(is (abs (- curr.0 other.0))
(abs (- curr.1 other.1))))
```
Output:

The output is one solution per line, each solution in the form `((row col) (row col) (row col) ...)`:

```arc> (nqueens 4)
((3 2) (2 0) (1 3) (0 1))
((3 1) (2 3) (1 0) (0 2))```

## Arturo

```result: new []

queens: function [n, i, a, b, c][
if? i < n [
loop 1..n 'j [
if all? @[
not? contains? a j
not? contains? b i+j
not? contains? c i-j
] ->
queens n, i+1, a ++ @[j], b ++ @[i+j], c ++ @[i-j]

]
]
else [
if n = size a ->
'result ++ @[a]
]
]

BoardSize: 6

queens BoardSize, 0, [], [], []
loop result 'solution [
loop solution 'col [

line: new repeat "-" BoardSize
line\[col-1]: `Q`
print line
]
print ""
]
```
Output:
```-Q----
---Q--
-----Q
Q-----
--Q---
----Q-

--Q---
-----Q
-Q----
----Q-
Q-----
---Q--

---Q--
Q-----
----Q-
-Q----
-----Q
--Q---

----Q-
--Q---
Q-----
-----Q
---Q--
-Q----```

## AWK

Inspired by Raymond Hettinger's Python solution, but builds the vector incrementally.

```#!/usr/bin/gawk -f
# Solve the Eight Queens Puzzle
#    Inspired by Raymond Hettinger [https://code.activestate.com/recipes/576647/]
#    Just the vector of row positions per column is kept,
#    and filled with all possibilities from left to right recursively,
#    then checked against the columns left from the current one:
#    - is a queen in the same row
#    - is a queen in the digonal
#    - is a queen in the reverse diagonal
BEGIN {
dim = ARGC < 2 ? 8 : ARGV
# make vec an array
vec = 0
# scan for a solution
if (tryqueen(1, vec, dim))
result(vec, dim)
else
print "No solution with " dim " queens."
}

# try if a queen can be set in column (col)
function tryqueen(col, vec, dim,        new)  {
for (new = 1; new <= dim; ++new) {
# check all previous columns
if (noconflict(new, col, vec, dim)) {
vec[col] = new
if (col == dim)
return 1
# must try next column(s)
if (tryqueen(col+1, vec, dim))
return 1
}
}
# all tested, failed
return 0
}

#  check if setting the queen (new) in column (col) is ok
#  by checking the previous colums conflicts
function noconflict(new, col, vec, dim,    j) {
for (j = 1; j < col; j++) {
if (vec[j] == new)
return 0    # same row
if (vec[j] == new - col + j)
return 0        # diagonal conflict
if (vec[j] == new + col - j)
return 0        # reverse diagonal conflict
}
# no test failed, no conflict
return 1
}

# print matrix
function result(vec, dim,    row, col, sep, lne) {
# print the solution vector
for (row  = 1; row <= dim; ++row)
printf " %d", vec[row]
print

# print a board matrix
for (row = 1; row <= dim; ++row) {
lne = "|"
for (col = 1; col <= dim; ++col) {
if (row == vec[col])
lne = lne "Q|"
else
lne = lne "_|"
}
print lne
}
}
```
Output:
``` 1 5 8 6 3 7 2 4
|Q|_|_|_|_|_|_|_|
|_|_|_|_|_|_|Q|_|
|_|_|_|_|Q|_|_|_|
|_|_|_|_|_|_|_|Q|
|_|Q|_|_|_|_|_|_|
|_|_|_|Q|_|_|_|_|
|_|_|_|_|_|Q|_|_|
|_|_|Q|_|_|_|_|_|
```

## ATS

```(* ****** ****** *)
//
// Solving N-queen puzzle
//
(* ****** ****** *)
//
// How to test:
// ./queens
// How to compile:
// patscc -DATS_MEMALLOC_LIBC -o queens queens.dats
//
(* ****** ****** *)
//
#include
//
#include
//
(* ****** ****** *)

fun
solutions(N:int) = let
//
fun
show
(
board: list0(int)
) : void =
(
list0_foreach<int>
( list0_reverse(board)
, lam(n) => ((N).foreach()(lam(i) => print_string(if i = n then " Q" else " _")); print_newline())
) ;
print_newline()
)
//
fun
safe
(
i: int, j: int, k: int, xs: list0(int)
) : bool =
(
case+ xs of
| nil0() => true
| cons0(x, xs) => x != i && x != j && x != k && safe(i, j+1, k-1, xs)
)
//
fun
loop
(
col: int, xs: list0(int)
) : void =
(N).foreach()
(
lam(i) =>
if
safe(i, i+1, i-1, xs)
then let
val xs = cons0(i, xs)
in
if col = N then show(xs) else loop(col+1, xs)
end // end of [then]
)
//
in
loop(1, nil0())
end // end of [solutions]

(* ****** ****** *)

val () = solutions(8)

(* ****** ****** *)

implement main0() = ()

(* ****** ****** *)

(* end of [queens.dats] *)```

## AutoHotkey

### Output to formatted Message box

Translation of: C
```;
; Post: http://www.autohotkey.com/forum/viewtopic.php?p=353059#353059
; Timestamp: 05/may/2010
;

MsgBox % funcNQP(5)
MsgBox % funcNQP(8)

Return

;~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
;
; ** USED VARIABLES **
;
; Global: All variables named Array[???]
;
; Function funcNPQ: nQueens , OutText , qIndex
;
; Function Unsafe: nIndex , Idx , Tmp , Aux
;
; Function PutBoard: Output , QueensN , Stc , xxx , yyy
;
;~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

funcNQP(nQueens)
{
Global
Array := -1
Local OutText , qIndex := 0
While ( qIndex >= 0 )
{
Array[%qIndex%]++
While ( (Array[%qIndex%] < nQueens) && Unsafe(qIndex) )
Array[%qIndex%]++
If ( Array[%qIndex%] < nQueens )
{
If ( qIndex < nQueens-1 )
qIndex++  ,  Array[%qIndex%] := -1
Else
PutBoard(OutText,nQueens)
}
Else
qIndex--
}
Return OutText
}

;------------------------------------------

Unsafe(nIndex)
{
Global
Local Idx := 1  ,  Tmp := 0  ,  Aux := Array[%nIndex%]
While ( Idx <= nIndex )
{
Tmp := "Array[" nIndex - Idx "]"
Tmp := % %Tmp%
If ( ( Tmp = Aux ) || ( Tmp = Aux-Idx ) || ( Tmp = Aux+Idx ) )
Return 1
Idx++
}
Return 0
}

;------------------------------------------

PutBoard(ByRef Output,QueensN)
{
Global
Static Stc = 0
Local xxx := 0 , yyy := 0
Output .= "`n`nSolution #" (++Stc) "`n"
While ( yyy < QueensN )
{
xxx := 0
While ( xxx < QueensN )
Output .= ( "|" ( ( Array[%yyy%] = xxx ) ? "Q" : "_" ) )  ,  xxx++
Output .= "|`n"  ,  yyy++
}
}
```

### Includes a solution browser GUI

This implementation supports N = 4..12 queens, and will find ALL solutions for each of the different sizes. The screenshot shows the first solution of 10 possible solutions for N = 5 queens.

```N := 5
Number: ; main entrance for different # of queens
SI := 1
Progress b2 w250 zh0 fs9, Calculating all solutions for %N% Queens ...
Gosub GuiCreate
Result := SubStr(Queens(N),2)
Progress Off
Gui Show,,%N%-Queens
StringSplit o, Result, `n
Fill: ; show solutions
GuiControl,,SI, %SI% / %o0%
Loop Parse, o%SI%, `,
{
C := A_Index
Loop %N%
GuiControl,,%C%_%A_Index% ; clear fields
GuiControl,,%C%_%A_LoopField%, r
}
Return ;-----------------------------------------------------------------------

Queens(N) {                                 ; Size of the board
Local c, O                              ; global array r
r1 := 1, c := 2, r2 := 3, O := ""       ; init: r%c% = row of Queen in column c

Right:                                  ; move to next column
If (c = N) {                        ; found solution
Loop %N%                        ; save row indices of Queens
O .= (A_Index = 1 ? "`n" : ",") r%A_Index%
GOTO % --c ? "Down" : "OUT"     ; for ALL solutions
}
c++, r%c% := 1                      ; next column, top row
GoTo % BAD(c) ? "Down" : "Right"
Down:                                   ; move down to next row
If (r%c% = N)
GoTo % --c ? "Down" : "OUT"
r%c%++                              ; row down
GoTo % BAD(c) ? "Down" : "Right"
OUT:
Return O
} ;----------------------------------------------------------------------------

BAD(c) { ; Check placed Queens against Queen in row r%c%, column c
Loop % c-1
If (r%A_Index% = r%c% || ABS(r%A_Index%-r%c%) = c-A_Index)
Return 1
} ;----------------------------------------------------------------------------

GuiCreate: ; Draw chess board
Gui Margin, 20, 15
Gui Font, s16, Marlett
Loop %N% {
C := A_Index
Loop %N% { ; fields
R := A_Index, X := 40*C-17, Y := 40*R-22
Gui Add, Progress, x%X% y%Y% w41 h41 Cdddddd, % 100*(R+C & 1) ;% shade fields
Gui Add, Text, x%X% y%Y% w41 h41 BackGroundTrans Border Center 0x200 v%C%_%R%
}
}
Gui Add, Button, x%x% w43 h25 gBF, 4 ; forth (default)
Gui Add, Button,xm yp w43 h25 gBF, 3 ; back

Gui Font, bold, Comic Sans MS
Gui Add, Text,% "x62 yp hp Center 0x200 vSI w" 40*N-80

Loop 9
Return ; ----------------------------------------------------------------------

Gui 1: +OwnDialogs
MsgBox, 64, About N-Queens, Many thanks ...
Return

Calculate: ; menu handler for calculations
Gui Destroy
GoTo Number ; -------------------------------------------------------------

BF:
SI := mod(SI+o0-2*(A_GuiControl=3), o0) + 1 ; left button text is "3"
GoTo Fill ; ----------------------------------------------------------------

GuiClose:
ExitApp
```

## BBC BASIC

The total number of solutions is displayed in the title bar and one solution is displayed. The code could be adapted to display a selected solution or multiple solutions.

```      Size% = 8
Cell% = 32
VDU 23,22,Size%*Cell%;Size%*Cell%;Cell%,Cell%,16,128+8,5
*font Arial Unicode MS,16
GCOL 3,11
FOR i% = 0 TO Size%-1 STEP 2
RECTANGLE FILL i%*Cell%*2,0,Cell%*2,Size%*Cell%*2
RECTANGLE FILL 0,i%*Cell%*2,Size%*Cell%*2,Cell%*2
NEXT
num% = FNqueens(Size%, Cell%)
SYS "SetWindowText", @hwnd%, "Total " + STR\$(num%) + " solutions"
REPEAT : WAIT 1 : UNTIL FALSE
END

DEF FNqueens(n%, s%)
LOCAL i%, j%, m%, p%, q%, r%, a%(), b%(), c%()
DIM a%(n%), b%(n%), c%(4*n%-2)
FOR i% = 1 TO DIM(a%(),1) : a%(i%) = i% : NEXT
m% = 0
i% = 1
j% = 0
r% = 2*n%-1
REPEAT
i% -= 1
j% += 1
p% = 0
q% = -r%
REPEAT
i% += 1
c%(p%) = 1
c%(q%+r%) = 1
SWAP a%(i%),a%(j%)
p% = i% - a%(i%) + n%
q% = i% + a%(i%) - 1
b%(i%) = j%
j% = i% + 1
UNTIL j% > n% OR c%(p%) OR c%(q%+r%)
IF c%(p%)=0 IF c%(q%+r%)=0 THEN
IF m% = 0 THEN
FOR p% = 1 TO n%
MOVE 2*s%*(a%(p%)-1)+6, 2*s%*p%+6
PRINT "♛";
NEXT
ENDIF
m% += 1
ENDIF
j% = b%(i%)
WHILE j% >= n% AND i% <> 0
REPEAT
SWAP a%(i%), a%(j%)
j% = j%-1
UNTIL j% < i%
i% -= 1
p% = i% - a%(i%) + n%
q% = i% + a%(i%) - 1
j% = b%(i%)
c%(p%) = 0
c%(q%+r%) = 0
ENDWHILE
UNTIL i% = 0
= m%
```

## BCPL

```// This can be run using Cintcode BCPL freely available from www.cl.cam.ac.uk/users/mr10.

GET "libhdr.h"

GLOBAL { count:ug; all  }

LET try(ld, row, rd) BE TEST row=all

THEN count := count + 1

ELSE { LET poss = all & ~(ld | row | rd)
WHILE poss DO
{ LET p = poss & -poss
poss := poss - p
try(ld+p << 1, row+p, rd+p >> 1)
}
}

LET start() = VALOF
{ all := 1

FOR i = 1 TO 16 DO
{ count := 0
try(0, 0, 0)
writef("Number of solutions to %i2-queens is %i7*n", i, count)
all := 2*all + 1
}

RESULTIS 0
}```

The following is a re-implementation of the algorithm given above but using the MC package that allows machine independent runtime generation of native machine code (currently only available for i386 machines). It runs about 25 times faster that the version given above.

```GET "libhdr.h"
GET "mc.h"

MANIFEST {
lo=1; hi=16
dlevel=#b0000

// Register mnemonics
ld    = mc_a
row   = mc_b
rd    = mc_c
poss  = mc_d
p     = mc_e
count = mc_f
}

LET start() = VALOF
{ // Load the dynamic code generation package
LET mcb = 0

UNLESS mcseg DO
{ writef("Trouble with MC package: mci386*n")
GOTO fin
}

// Create an MC instance for hi functions with a data space
// of 10 words and code space of 40000
mcb := mcInit(hi, 10, 40000)

UNLESS mcb DO
{ writef("Unable to create an mci386 instance*n")
GOTO fin
}

mc := 0          // Currently no selected MC instance
mcSelect(mcb)

mcK(mc_debug, dlevel) // Set the debugging level

FOR n = lo TO hi DO
{ mcComment("*n*n// Code for a %nx%n board*n", n, n)
gencode(n) // Compile the code for an nxn board
}

mcF(mc_end) // End of code generation

writef("Code generation complete*n")

FOR n = lo TO hi DO
{ LET k = mcCall(n)
writef("Number of solutions to %i2-queens is %i9*n", n, k)
}

fin:
IF mc    DO mcClose()

writef("*n*nEnd of run*n")
}

AND gencode(n) BE
{ LET all = (1<<n) - 1
mcKKK(mc_entry, n, 3, 0)

mcRK(mc_mv, ld,    0)
mcRK(mc_mv, row,   0)
mcRK(mc_mv, rd,    0)
mcRK(mc_mv, count, 0)

cmpltry(1, n, all)        // Compile the outermost call of try

mcRR(mc_mv, mc_a, count)  // return count
mcF(mc_rtn)
mcF(mc_endfn)
}

AND cmpltry(i, n, all) BE
{ LET L = mcNextlab()

mcComment("*n// Start of code from try(%n, %n, %n)*n", i, n, all)

mcRR(mc_mv,  poss, ld)         // LET poss = (~(ld | row | rd)) & all
mcRR(mc_or,  poss, row)
mcRR(mc_or,  poss, rd)
mcR (mc_not, poss)
mcRK(mc_and, poss, all)

mcRK(mc_cmp, poss, 0)          // IF poss DO
TEST n-i<=2
THEN mcJS(mc_jeq, L)           // (use a short jump if near the last row)
ELSE mcJL(mc_jeq, L)

TEST i=n
THEN { // We can place a queen in the final row.
mcR(mc_inc,  count)     //   count := count+1
}
ELSE { // We can place queen(s) in a non final row.
LET M = mcNextlab()

mcL (mc_lab,  M)        // { Start of REPEATWHILE loop

mcRR(mc_mv,   p, poss)  //   LET p = poss & -poss
mcR (mc_neg,  p)
mcRR(mc_and,  p, poss)  //   // p is a valid queens position
mcRR(mc_sub,  poss, p)  //   poss := poss - p

mcR (mc_push, ld)       //   Save current state
mcR (mc_push, row)
mcR (mc_push, rd)
mcR (mc_push, poss)
//   Call try((ld+p)<<1, row+p, (rd+p)>>1)
mcRK(mc_lsh,  ld,  1)   //   ld  := (ld+p)<<1
mcRR(mc_add,  row, p)   //   row := row+p
mcRK(mc_rsh,  rd,  1)   //   rd  := (rd+p)>>1

cmpltry(i+1, n, all)    //   Compile code for row i+1

mcR (mc_pop,  poss)     //   Restore the state
mcR (mc_pop,  rd)
mcR (mc_pop,  row)
mcR (mc_pop,  ld)

mcRK(mc_cmp,  poss, 0)
mcJL(mc_jne, M)         // } REPEATWHILE poss
}

mcL(mc_lab, L)
mcComment("// End   of code from try(%n, %n, %n)*n*n",
i, n, all)
}```

## Befunge

This algorithm works with any board size from 4 upwards.

```<+--XX@_v#!:-1,+55,g\1\$_:00g2%-0vv:,+55<&,,,,,,"Size: "
"| Q"\$\$\$>:01p:2%!00g0>>^<<!:-1\<1>00p::2%-:40p2/50p2*1+
!77**48*+31p\:1\g,::2\g:,\3\g,,^g>0g++40g%40g\-\40g\`*-
2g05\**!!%6g04-g052!:`\g05::-1/2<^4*2%g05\+*+1*!!%6g04-
```
Output:
```Size: 8

+---+---+---+---+---+---+---+---+
|   |   |   |   | Q |   |   |   |
+---+---+---+---+---+---+---+---+
|   |   | Q |   |   |   |   |   |
+---+---+---+---+---+---+---+---+
| Q |   |   |   |   |   |   |   |
+---+---+---+---+---+---+---+---+
|   |   |   |   |   |   | Q |   |
+---+---+---+---+---+---+---+---+
|   | Q |   |   |   |   |   |   |
+---+---+---+---+---+---+---+---+
|   |   |   |   |   |   |   | Q |
+---+---+---+---+---+---+---+---+
|   |   |   |   |   | Q |   |   |
+---+---+---+---+---+---+---+---+
|   |   |   | Q |   |   |   |   |
+---+---+---+---+---+---+---+---+```

## Bracmat

```(   ( printBoard
=   board M L x y S R row line
.   :?board
& !ups:? [?M
&   whl
' ( !arg:(?x.?y) ?arg
& !M:?L
& :?row:?line
&   whl
' ( !L+-1:~<0:?L
& !x+1:~>!M:?x
& "---+" !line:?line
& "   |" !row:?row
)
& "---+" !line:?line
& " Q |" !row:?row
&   whl
' ( !L+-1:~<0:?L
& "---+" !line:?line
& "   |" !row:?row
)
& "\n|" !row "\n+" !line !board:?board
)
& str\$("\n+" !line !board)
)
( queens
=   hor ver up down ups downs a z A Z x y Q
.   !arg:(?hor.?ver.?ups.?downs.?Q)
&   !ver
: (
& 1+!solutions:?solutions
{ Comment the line below if you only want a count. }
& out\$(str\$("\nsolution " !solutions) printBoard\$!Q)
& ~  { Fail! (and backtrack to find more solutions)}
|   #%?y
( ?z
&   !hor
:   ?A
#%?x
( ?Z
& !x+!y:?up
& !x+-1*!y:?down
& ~(!ups:? !up ?)
& ~(!downs:? !down ?)
&   queens
\$ ( !A !Z
. !z
. !up !ups
. !down !downs
. (!x.!y) !Q
)
)
)
)
)
& 0:?solutions
& 1 2 3 4 5 6 7 8:?H:?V   {You can edit this line to find solutions for other sizes.}
& ( queens\$(!H.!V...)
| out\$(found !solutions solutions)
)
);```
Output:
(tail)
```solution 91

+---+---+---+---+---+---+---+---+
|   |   |   |   |   |   |   | Q |
+---+---+---+---+---+---+---+---+
|   |   | Q |   |   |   |   |   |
+---+---+---+---+---+---+---+---+
| Q |   |   |   |   |   |   |   |
+---+---+---+---+---+---+---+---+
|   |   |   |   |   | Q |   |   |
+---+---+---+---+---+---+---+---+
|   | Q |   |   |   |   |   |   |
+---+---+---+---+---+---+---+---+
|   |   |   |   | Q |   |   |   |
+---+---+---+---+---+---+---+---+
|   |   |   |   |   |   | Q |   |
+---+---+---+---+---+---+---+---+
|   |   |   | Q |   |   |   |   |
+---+---+---+---+---+---+---+---+

solution 92

+---+---+---+---+---+---+---+---+
|   |   |   |   |   |   |   | Q |
+---+---+---+---+---+---+---+---+
|   |   |   | Q |   |   |   |   |
+---+---+---+---+---+---+---+---+
| Q |   |   |   |   |   |   |   |
+---+---+---+---+---+---+---+---+
|   |   | Q |   |   |   |   |   |
+---+---+---+---+---+---+---+---+
|   |   |   |   |   | Q |   |   |
+---+---+---+---+---+---+---+---+
|   | Q |   |   |   |   |   |   |
+---+---+---+---+---+---+---+---+
|   |   |   |   |   |   | Q |   |
+---+---+---+---+---+---+---+---+
|   |   |   |   | Q |   |   |   |
+---+---+---+---+---+---+---+---+
found 92 solutions```

## C

C99, compiled with `gcc -std=c99 -Wall`. Take one commandline argument: size of board, or default to 8. Shows the board layout for each solution.
```#include <stdio.h>
#include <stdlib.h>

int count = 0;
void solve(int n, int col, int *hist)
{
if (col == n) {
printf("\nNo. %d\n-----\n", ++count);
for (int i = 0; i < n; i++, putchar('\n'))
for (int j = 0; j < n; j++)
putchar(j == hist[i] ? 'Q' : ((i + j) & 1) ? ' ' : '.');

return;
}

#	define attack(i, j) (hist[j] == i || abs(hist[j] - i) == col - j)
for (int i = 0, j = 0; i < n; i++) {
for (j = 0; j < col && !attack(i, j); j++);
if (j < col) continue;

hist[col] = i;
solve(n, col + 1, hist);
}
}

int main(int n, char **argv)
{
if (n <= 1 || (n = atoi(argv)) <= 0) n = 8;
int hist[n];
solve(n, 0, hist);
}
```
Similiar to above, but using bits to save board configurations and quite a bit faster:
```#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>

typedef uint32_t uint;
uint full, *qs, count = 0, nn;

void solve(uint d, uint c, uint l, uint r)
{
uint b, a, *s;
if (!d) {
count++;
#if 0
printf("\nNo. %d\n===========\n", count);
for (a = 0; a < nn; a++, putchar('\n'))
for (b = 0; b < nn; b++, putchar(' '))
putchar(" -QQ"[((b == qs[a])<<1)|((a + b)&1)]);
#endif
return;
}

a = (c | (l <<= 1) | (r >>= 1)) & full;
if (a != full)
for (*(s = qs + --d) = 0, b = 1; b <= full; (*s)++, b <<= 1)
if (!(b & a)) solve(d, b|c, b|l, b|r);
}

int main(int n, char **argv)
{
if (n <= 1 || (nn = atoi(argv)) <= 0) nn = 8;

qs = calloc(nn, sizeof(int));
full = (1U << nn) - 1;

solve(nn, 0, 0, 0);
printf("\nSolutions: %d\n", count);
return 0;
}
```

Take that and unwrap the recursion, plus some heavy optimizations, and we have a very fast and very unreadable solution:

```#include <stdio.h>
#include <stdlib.h>

typedef unsigned int uint;
uint count = 0;

#define ulen sizeof(uint) * 8

/* could have defined as int solve(...), but void may have less
chance to confuse poor optimizer */
void solve(int n)
{
int cnt = 0;
const uint full = -(int)(1 << (ulen - n));
register uint bits, pos, *m, d, e;

uint b0, b1, l, r, c, mm = {0};
n -= 3;
/* require second queen to be left of the first queen, so
we ever only test half of the possible solutions. This
is why we can't handle n=1 here */
for (b0 = 1U << (ulen - n - 3); b0; b0 <<= 1) {
for (b1 = b0 << 2; b1; b1 <<= 1) {
d = n;
/* c: columns occupied by previous queens.
l: columns attacked by left diagonals
r: by right diagnoals */
c[n] = b0 | b1;
l[n] = (b0 << 2) | (b1 << 1);
r[n] = (b0 >> 2) | (b1 >> 1);

/* availabe columns on current row. m is stack */
bits = *(m = mm + 1) = full & ~(l[n] | r[n] | c[n]);

while (bits) {
/* d: depth, aka row. counting backwards
because !d is often faster than d != n */
while (d) {
/* pos is right most nonzero bit */
pos = -(int)bits & bits;

/* mark bit used. only put current bits
on stack if not zero, so backtracking
will skip exhausted rows (because reading
stack variable is sloooow compared to
registers) */
if ((bits &= ~pos))
*m++ = bits | d;

/* faster than l[d+1] = l[d]... */
e = d--;
l[d] = (l[e] | pos) << 1;
r[d] = (r[e] | pos) >> 1;
c[d] =  c[e] | pos;

bits = full & ~(l[d] | r[d] | c[d]);

if (!bits) break;
if (!d) { cnt++; break; }
}
/* Bottom of stack m is a zero'd field acting
as sentinel.  When saving to stack, left
27 bits are the available columns, while
right 5 bits is the depth. Hence solution
is limited to size 27 board -- not that it
matters in foreseeable future. */
d = (bits = *--m) & 31U;
bits &= ~31U;
}
}
}
count = cnt * 2;
}

int main(int c, char **v)
{
int nn;
if (c <= 1 || (nn = atoi(v)) <= 0) nn = 8;

if (nn > 27) {
fprintf(stderr, "Value too large, abort\n");
exit(1);
}

/* Can't solve size 1 board; might as well skip 2 and 3 */
if (nn < 4) count = nn == 1;
else	    solve(nn);

printf("\nSolutions: %d\n", count);
return 0;
}
```

A slightly cleaned up version of the code above where some optimizations were redundant. This version is also further optimized, and runs about 15% faster than the one above on modern compilers:

```#include <stdio.h>
#define MAXN 31

int nqueens(int n)
{
int q0,q1;
int cols[MAXN], diagl[MAXN], diagr[MAXN], posibs[MAXN]; // Our backtracking 'stack'
int num=0;
//
// The top level is two fors, to save one bit of symmetry in the enumeration by forcing second queen to
// be AFTER the first queen.
//
for (q0=0; q0<n-2; q0++) {
for (q1=q0+2; q1<n; q1++){
int bit0 = 1<<q0;
int bit1 = 1<<q1;
int d=0; // d is our depth in the backtrack stack
cols = bit0 | bit1 | (-1<<n); // The -1 here is used to fill all 'coloumn' bits after n ...
diagl= (bit0<<1 | bit1)<<1;
diagr= (bit0>>1 | bit1)>>1;

//  The variable posib contains the bitmask of possibilities we still have to try in a given row ...
int posib = ~(cols | diagl | diagr);

while (d >= 0) {
while(posib) {
int bit = posib & -posib; // The standard trick for getting the rightmost bit in the mask
int ncols= cols[d] | bit;
int ndiagl = (diagl[d] | bit) << 1;
int ndiagr = (diagr[d] | bit) >> 1;
int nposib = ~(ncols | ndiagl | ndiagr);
posib^=bit; // Eliminate the tried possibility.

// The following is the main additional trick here, as recognizing solution can not be done using stack level (d),
// since we save the depth+backtrack time at the end of the enumeration loop. However by noticing all coloumns are
// filled (comparison to -1) we know a solution was reached ...
// Notice also that avoiding an if on the ncols==-1 comparison is more efficient!
num += ncols==-1;

if (nposib) {
if (posib) { // This if saves stack depth + backtrack operations when we passed the last possibility in a row.
posibs[d++] = posib; // Go lower in stack ..
}
cols[d] = ncols;
diagl[d] = ndiagl;
diagr[d] = ndiagr;
posib = nposib;
}
}
posib = posibs[--d]; // backtrack ...
}
}
}
return num*2;
}

main(int ac , char **av)
{
if(ac != 2) {
printf("usage: nq n\n");
return 1;
}
int n = atoi(av);
if(n<1 || n > MAXN) {
printf("n must be between 2 and 31!\n");
}
printf("Number of solution for %d is %d\n",n,nqueens(n));
}
```

## C#

### Roger Hui (1981) Algorithm

From Hui, Roger, The N Queens Problem, APL Quote-Quad, Volume 11, Number 3, 1981-03:-

"In a solution, each possible row (column) index must appear exactly once: an index occurring more than once means that two queens are on the same row (column); and the absence of an index means that some other index must occur more than once. Hence, we can specify an arrangement as a permutation of ⍳n , which are the column indices, with the row indices understood to be ⍳n . With this, the number of possibilities is reduced from n!n×n to !n . It remains to eliminate arrangements having two queens on the same diagonal.

If two queens occupy the same diagonal, the line connecting them has slope 1 or ¯1 . Conversely, if the line connecting two queens has slope 1 or ¯1 , the two queens share a diagonal. Therefore, we seek to eliminate all permutations specifying a pair of queens where ((change in y) ÷ (change in x)) ∊ 1 ¯1 , or (|change in y) = (|change in x)"

Translation of: J
Works with: C# version 7
```using System.Collections.Generic;
using static System.Linq.Enumerable;
using static System.Console;
using static System.Math;

namespace N_Queens
{
static class Program
{
static void Main(string[] args)
{
var n = 8;
var cols = Range(0, n);
var combs = cols.Combinations(2).Select(pairs=> pairs.ToArray());
var solved = from v in cols.Permutations().Select(p => p.ToArray())
where combs.All(c => Abs(v[c] - v[c]) != Abs(c - c))
select v;

WriteLine(\$"{n}-queens has {solved.Count()} solutions");
WriteLine("Position is row, value is column:-");
var first = string.Join(" ", solved.First());
WriteLine(\$"First Solution: {first}");
}

//Helpers
public static IEnumerable<IEnumerable<T>> Permutations<T>(this IEnumerable<T> values)
{
if (values.Count() == 1)
return values.ToSingleton();

return values.SelectMany(v => Permutations(values.Except(v.ToSingleton())), (v, p) => p.Prepend(v));
}

public static IEnumerable<IEnumerable<T>> Combinations<T>(this IEnumerable<T> seq) =>
seq.Aggregate(Empty<T>().ToSingleton(), (a, b) => a.Concat(a.Select(x => x.Append(b))));

public static IEnumerable<IEnumerable<T>> Combinations<T>(this IEnumerable<T> seq, int numItems) =>
seq.Combinations().Where(s => s.Count() == numItems);

public static IEnumerable<T> ToSingleton<T>(this T item) { yield return item; }
}
}
```

Output

```8-queens has 92 solutions
Position is row, value is column:-
First Solution: 0 4 7 5 2 6 1 3
```

### Hettinger Algorithm

Compare this to the Hettinger solution used in the first Python answer. The logic is similar but the diagonal calculation is different and more expensive computationally (Both suffer from being unable to eliminate permutation prefixes that are invalid e.g. 0 1 ...)

```using System.Collections.Generic;
using static System.Linq.Enumerable;
using static System.Console;
using static System.Math;

namespace N_Queens
{
static class Program
{
static void Main(string[] args)
{
var n = 8;
var cols = Range(0, n);
var solved = from v in cols.Permutations().Select(p => p.ToArray())
where n == (from i in cols select v[i]+i).Distinct().Count()
where n == (from i in cols select v[i]-i).Distinct().Count()
select v;

WriteLine(\$"{n}-queens has {solved.Count()} solutions");
WriteLine("Position is row, value is column:-");
var first = string.Join(" ", solved.First());
WriteLine(\$"First Solution: {first}");
}

//Helpers from https://gist.github.com/martinfreedman/139dd0ec7df4737651482241e48b062f

public static IEnumerable<IEnumerable<T>> Permutations<T>(this IEnumerable<T> values)
{
if (values.Count() == 1)
return values.ToSingleton();

return values.SelectMany(v => Permutations(values.Except(v.ToSingleton())), (v, p) => p.Prepend(v));
}

public static IEnumerable<T> ToSingleton<T>(this T item) { yield return item; }
}
}
```

### Amb solution

This uses the second version of the Amb C# class in the Amb challenge. Really that is not McCarthy's Amb (Ambiguous function) and here it is used just as a simple general interface by lambdas to a standalone backtrack algorithm. Due to the specification of the Amb challenge, this, ironically (given the notion of ambiguous functions), only produces one solution not 92. It is trivial to update Amb (might be better called a backtracker rather than Amb too) but here it is just used to show how easy it is to go from a generate and prune Linq solution to a backtrack solution. The Linq filters becoming "amb" requirements.

Works with: C# version 7.1
```using static System.Linq.Enumerable;
using static System.Console;

namespace N_Queens
{
static class Program
{
static void Main(string[] args)
{
var n = 8;
var domain = Range(0, n).ToArray();

var amb = new Amb.Amb();
var queens = domain.Select(_ => amb.Choose(domain)).ToArray();
amb.Require(() => n == queens.Select(q=> q.Value).Distinct().Count());
amb.Require(() => n == domain.Select(i=> i + queens[i].Value).Distinct().Count());
amb.Require(() => n == domain.Select(i=> i - queens[i].Value).Distinct().Count());

if (amb.Disambiguate())
{
WriteLine("Position is row, value is column:-");
WriteLine(string.Join(" ", queens.AsEnumerable()));
}
else
WriteLine("amb is angry");
}
}
}
```

## C++

```// Much shorter than the version below;
// uses C++11 threads to parallelize the computation; also uses backtracking
// Outputs all solutions for any table size
#include <vector>
#include <iostream>
#include <iomanip>
#include <future>

// Print table. 'pos' is a vector of positions – the index in pos is the row,
// and the number at that index is the column where the queen is placed.
static void print(const std::vector<int> &pos)
{
for (int i = 0; i < pos.size(); i++) {
std::cout << std::setw(3) << char('a' + i);
}

std::cout << '\n';

for (int row = 0; row < pos.size(); row++) {
int col = pos[row];
std::cout << row + 1 << std::setw(3 * col + 3) << " # ";
std::cout << '\n';
}

std::cout << "\n\n";
}

static bool threatens(int row_a, int col_a, int row_b, int col_b)
{
return row_a == row_b // same row
or col_a == col_b // same column
or std::abs(row_a - row_b) == std::abs(col_a - col_b); // diagonal
}

// the i-th queen is in the i-th row
// we only check rows up to end_idx
// so that the same function can be used for backtracking and checking the final solution
static bool good(const std::vector<int> &pos, int end_idx)
{
for (int row_a = 0; row_a < end_idx; row_a++) {
for (int row_b = row_a + 1; row_b < end_idx; row_b++) {
int col_a = pos[row_a];
int col_b = pos[row_b];
if (threatens(row_a, col_a, row_b, col_b)) {
return false;
}
}
}

return true;
}

static std::mutex print_count_mutex; // mutex protecting 'n_sols'
static int n_sols = 0; // number of solutions

// recursive DFS backtracking solver
static void n_queens(std::vector<int> &pos, int index)
{
// if we have placed a queen in each row (i. e. we are at a leaf of the search tree), check solution and return
if (index >= pos.size()) {
if (good(pos, index)) {
std::lock_guard<std::mutex> lock(print_count_mutex);
print(pos);
n_sols++;
}

return;
}

// backtracking step
if (not good(pos, index)) {
return;
}

// optimization: the first level of the search tree is parallelized
if (index == 0) {
std::vector<std::future<void>> fts;
for (int col = 0; col < pos.size(); col++) {
pos[index] = col;
auto ft = std::async(std::launch::async, [=]{ auto cpos(pos); n_queens(cpos, index + 1); });
fts.push_back(std::move(ft));
}

for (const auto &ft : fts) {
ft.wait();
}
} else { // deeper levels are not
for (int col = 0; col < pos.size(); col++) {
pos[index] = col;
n_queens(pos, index + 1);
}
}
}

int main()
{
std::vector<int> start(12); // 12: table size
n_queens(start, 0);
std::cout << n_sols << " solutions found.\n";
return 0;
}
```
Output:
Output for N = 4
```  a  b  c  d
1    #
2          #
3 #
4       #

a  b  c  d
1       #
2 #
3          #
4    #       ```
```// A straight-forward brute-force C++ version with formatted output,
// eschewing obfuscation and C-isms, producing ALL solutions, which
// works on any OS with a text terminal.
//
// Two basic optimizations are applied:
//
//   It uses backtracking to only construct potentially valid solutions.
//
//   It only computes half the solutions by brute -- once we get the
//   queen halfway across the top row, any remaining solutions must be
//   reflections of the ones already computed.
//
// This is a bare-bones example, without any progress feedback or output
// formatting controls, which a more complete program might provide.
//
// Beware that computing anything larger than N=14 might take a while.
// (Time gets exponentially worse the higher the number.)

// Copyright 2014 Michael Thomas Greer

#include <algorithm>
#include <ciso646>
#include <iomanip>
#include <iostream>
#include <set>
#include <sstream>
#include <stdexcept>
#include <string>
#include <vector>

// ///////////////////////////////////////////////////////////////////////////
struct queens
/////////////////////////////////////////////////////////////////////////// //
{
// TYPES -------------------------------------------------------------------

// A row or column index. (May be signed or unsigned.)
//
typedef signed char index_type;

// A 'solution' is a row --> column lookup of queens on the board.
//
// It has lexicographical order and can be transformed with a variety of
// reflections, which, when properly combined, produce all possible
// orientations of a solution.
//
struct solution_type: std::vector <index_type>
{
typedef std::vector <index_type> base_type;

// constructors  . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
solution_type( std::size_t N          ): base_type( N, -1 ) { }
solution_type( const solution_type& s ): base_type( s     ) { }

// compare . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
bool operator < ( const solution_type& s ) const
{
auto mm = std::mismatch( begin(), end(), s.begin() );
return (mm.first != end()) and (*mm.first < *mm.second);
}

// transformations . . . . . . . . . . . . . . . . . . . . . . . . . . . .
void vflip() { std::reverse( begin(), end() ); }

void hflip() { for (auto& x : *this) x = size() - 1 - x; }

void transpose()
{
solution_type result( size() );
for (index_type y = 0; (std::size_t)y < size(); y++)
result[ (*this)[ y ] ] = y;
swap( result );
}
};

// MEMBER VALUES -----------------------------------------------------------

const int                N;
std::set <solution_type> solutions;

// SOLVER ------------------------------------------------------------------

queens( int N = 8 ):
N( (N < 0) ? 0 : N )
{
// Row by row we create a potentially valid solution.
// If a queen can be placed in a valid spot by the time
// we get to the last row, then we've found a solution.

solution_type solution( N );
index_type row = 0;
while (true)
{
// Advance the queen along the row
++solution[ row ];

// (If we get past halfway through the first row, we're done.)
if ((row == 0) and (solution[ 0 ] > N/2)) break;

if (solution[ row ] < N)
{
// If the queen is in a good spot...
if (ok( solution, row, solution[ row ] ))
{
// ...and we're on the last row
if (row == N-1)
{
// Add the solution we found plus all it's reflections
solution_type
s = solution;  solutions.insert( s );
s.vflip();     solutions.insert( s );
s.hflip();     solutions.insert( s );
s.vflip();     solutions.insert( s );
s.transpose(); solutions.insert( s );
s.vflip();     solutions.insert( s );
s.hflip();     solutions.insert( s );
s.vflip();     solutions.insert( s );
}
// otherwise begin marching a queen along the next row
else solution[ ++row ] = -1;
}

// When we get to the end of a row's columns then
// we need to backup a row and continue from there.
}
else --row;
}
}

// HELPER ------------------------------------------------------------------
// This routine helps the solver by identifying column locations
// that do not conflict with queens already placed in prior rows.

bool ok( const solution_type& columns, index_type row, index_type column )
{
for (index_type r = 0; r < row; r++)
{
index_type c         = columns[ r ];
index_type delta_row = row - r;
index_type delta_col = (c < column) ? (column - c) : (c - column);

if ((c == column) or (delta_row == delta_col))
return false;
}
return true;
}

// OUTPUT A SINGLE SOLUTION ------------------------------------------------
//
// Formatted as (for example):
//
//   d1 b2 g3 c4 f5 h6 e7 a8
//   Q - - - - - - -
//   - - - - Q - - -
//   - - - - - - - Q
//   - - - - - Q - -
//   - - Q - - - - -
//   - - - - - - Q -
//   - Q - - - - - -
//   - - - Q - - - -
//
friend
std::ostream&
operator << ( std::ostream& outs, const queens::solution_type& solution )
{
static const char* squares[] = { "- ", "Q " };
index_type N = solution.size();

// Display the queen positions
for (auto n = N; n--; )
outs << (char)('a' + solution[ n ]) << (N - n) << " ";

// Display the board
for (auto queen : solution)
{
outs << "\n";
for (index_type col = 0; col < N; col++)
outs << squares[ col == queen ];
}
return outs;
}

// OUTPUT ALL SOLUTIONS ----------------------------------------------------
//
// Display "no solutions" or "N solutions" followed by
// each individual solution, separated by blank lines.

friend
std::ostream&
operator << ( std::ostream& outs, const queens& q )
{
if (q.solutions.empty()) outs << "no";
else                     outs << q.solutions.size();
outs << " solutions";

std::size_t n = 1;
for (auto solution : q.solutions)
{
outs << "\n\n#" << n++ << "\n" << solution;
}

return outs;
}
};

/* ///////////////////////////////////////////////////////////////////////////
string_to <type> ( x )
/////////////////////////////////////////////////////////////////////////// */

template <typename T>
T string_to( const std::string& s )
{
T result;
std::istringstream ss( s );
ss >> result;
if (!ss.eof()) throw std::runtime_error( "to_string(): invalid conversion" );
return result;
}

template <typename T, T default_value>
T string_to( const std::string& s )
{
try { return string_to <T> ( s ); }
catch (...) { return default_value; }
}

/* ///////////////////////////////////////////////////////////////////////////
main program
/////////////////////////////////////////////////////////////////////////// */

int usage( const std::string& name )
{
std::cerr <<
"usage:\n  " << name << " 8\n\n"
""
"Solve the N-Queens problem, brute-force,\n"
"and show all solutions for an 8x8 board.\n\n"
""
"(Specify a value other than 8 for the board size you want.)\n";
return 1;
}

int main( int argc, char** argv )
{
signed N =
(argc < 2) ? 8 :
(argc > 2) ? 0 : string_to <signed, 0> ( argv[ 1 ] );

if (N <= 0) return usage( argv[ 0 ] );

std::cout << queens( N ) << "\n";
}
```
Output:
for N=4
```2 solutions

#1
c1 a2 d3 b4
- Q - -
- - - Q
Q - - -
- - Q -

#2
b1 d2 a3 c4
- - Q -
Q - - -
- - - Q
- Q - -
```

### Alternate version

Windows-only

```#include <windows.h>
#include <iostream>
#include <string>

//--------------------------------------------------------------------------------------------------
using namespace std;

//--------------------------------------------------------------------------------------------------
class point
{
public:
int x, y;
point(){ x = y = 0; }
void set( int a, int b ){ x = a; y = b; }
};
//--------------------------------------------------------------------------------------------------
class nQueens
{
public:
void solve( int c )
{
_count = c; int len = ( c + 1 ) * ( c + 1 ); _queens = new bool[len]; memset( _queens, 0, len );
_cl = new bool[c]; memset( _cl, 0, c ); _ln = new bool[c]; memset( _ln, 0, c );
point pt; pt.set( rand() % c, rand() % c ); putQueens( pt, c ); displayBoard();
delete [] _queens; delete [] _ln; delete [] _cl;
}

private:
void displayBoard()
{
system( "cls" ); string t = "+---+", q = "| Q |", s = "|   |";
COORD c = { 0, 0 }; HANDLE h = GetStdHandle( STD_OUTPUT_HANDLE );
for( int y = 0, cy = 0; y < _count; y++ )
{
int yy = y * _count;
for( int x = 0; x < _count; x++ )
{
SetConsoleCursorPosition( h, c ); cout << t;
c.Y++; SetConsoleCursorPosition( h, c );
if( _queens[x + yy] ) cout << q; else cout << s;
c.Y++; SetConsoleCursorPosition( h, c );
cout << t; c.Y = cy; c.X += 4;
}
cy += 2; c.X = 0; c.Y = cy;
}
}

bool checkD( int x, int y, int a, int b )
{
if( x < 0 || y < 0 || x >= _count || y >= _count ) return true;
if( _queens[x + y * _count] ) return false;
if( checkD( x + a, y + b, a, b ) ) return true;
return false;
}

bool check( int x, int y )
{
if( _ln[y] || _cl[x] )        return false;
if( !checkD( x, y, -1, -1 ) ) return false;
if( !checkD( x, y,  1, -1 ) ) return false;
if( !checkD( x, y, -1,  1 ) ) return false;
if( !checkD( x, y,  1,  1 ) ) return false;
return true;
}

bool putQueens( point pt, int cnt )
{
int it = _count;
while( it )
{
if( !cnt ) return true;
if( check( pt.x, pt.y ) )
{
_queens[pt.x + pt.y * _count] = _cl[pt.x] = _ln[pt.y] = true;
point tmp = pt; if( ++tmp.x >= _count ) tmp.x = 0; if( ++tmp.y >= _count ) tmp.y = 0;
if( putQueens( tmp, cnt - 1 ) ) return true;
_queens[pt.x + pt.y * _count] = _cl[pt.x] = _ln[pt.y] = false;
}
if( ++pt.x >= _count ) pt.x = 0;
it--;
}
return false;
}

int          _count;
bool*        _queens, *_ln, *_cl;
};
//--------------------------------------------------------------------------------------------------
int main( int argc, char* argv[] )
{
nQueens n; int nq;
while( true )
{
system( "cls" ); cout << "Enter board size bigger than 3 (0 - 3 to QUIT): "; cin >> nq;
if( nq < 4 ) return 0; n.solve( nq ); cout << endl << endl;
system( "pause" );
}
return  0;
}
//--------------------------------------------------------------------------------------------------
```
Output:
```+---+---+---+---+---+   +---+---+---+---+---+---+---+---+   +---+---+---+---+---+---+---+---+---+---+---+
| Q |   |   |   |   |   |   |   |   | Q |   |   |   |   |   |   |   |   |   |   |   |   | Q |   |   |   |
+---+---+---+---+---+   +---+---+---+---+---+---+---+---+   +---+---+---+---+---+---+---+---+---+---+---+
|   |   | Q |   |   |   |   | Q |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   | Q |   |
+---+---+---+---+---+   +---+---+---+---+---+---+---+---+   +---+---+---+---+---+---+---+---+---+---+---+
|   |   |   |   | Q |   |   |   |   |   |   |   |   | Q |   |   |   |   | Q |   |   |   |   |   |   |   |
+---+---+---+---+---+   +---+---+---+---+---+---+---+---+   +---+---+---+---+---+---+---+---+---+---+---+
|   | Q |   |   |   |   |   |   |   |   |   | Q |   |   |   |   | Q |   |   |   |   |   |   |   |   |   |
+---+---+---+---+---+   +---+---+---+---+---+---+---+---+   +---+---+---+---+---+---+---+---+---+---+---+
|   |   |   | Q |   |   | Q |   |   |   |   |   |   |   |   |   |   |   |   | Q |   |   |   |   |   |   |
+---+---+---+---+---+   +---+---+---+---+---+---+---+---+   +---+---+---+---+---+---+---+---+---+---+---+
|   |   | Q |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   | Q |
+---+---+---+---+---+---+---+---+   +---+---+---+---+---+---+---+---+---+---+---+
|   |   |   |   | Q |   |   |   |   | Q |   |   |   |   |   |   |   |   |   |   |
+---+---+---+---+---+---+---+---+   +---+---+---+---+---+---+---+---+---+---+---+
|   |   |   |   |   |   | Q |   |   |   |   | Q |   |   |   |   |   |   |   |   |
+---+---+---+---+---+---+---+---+   +---+---+---+---+---+---+---+---+---+---+---+
|   |   |   |   |   | Q |   |   |   |   |   |
+---+---+---+---+---+---+---+---+---+---+---+
|   |   |   |   |   |   |   |   | Q |   |   |
+---+---+---+---+---+---+---+---+---+---+---+
|   |   |   |   |   |   | Q |   |   |   |   |
+---+---+---+---+---+---+---+---+---+---+---+
```

Version using Heuristics - explained here: Solution_construction

```#include <windows.h>
#include <iostream>
#include <string>
#include <vector>
#include <algorithm>

//--------------------------------------------------------------------------------------------------
using namespace std;

//--------------------------------------------------------------------------------------------------
typedef unsigned int uint;

//--------------------------------------------------------------------------------------------------
class nQueens_Heuristic
{
public:
void solve( uint n ) { makeList( n ); drawBoard( n ); }

private:
void drawBoard( uint n )
{
system( "cls" ); string t = "+---+", q = "| Q |", s = "|   |";
COORD c = { 0, 0 }; HANDLE h = GetStdHandle( STD_OUTPUT_HANDLE );
uint w = 0;
for( uint y = 0, cy = 0; y < n; y++ )
{
for( uint x = 0; x < n; x++ )
{
SetConsoleCursorPosition( h, c ); cout << t;
c.Y++; SetConsoleCursorPosition( h, c );
if( x + 1 == solution[w] ) cout << q; else cout << s;
c.Y++; SetConsoleCursorPosition( h, c );
cout << t; c.Y = cy; c.X += 4;
}
cy += 2; c.X = 0; c.Y = cy; w++;
}
solution.clear(); odd.clear(); evn.clear();
}

void makeList( uint n )
{
uint r = n % 6;
for( uint x = 1; x <= n; x++ )
{
if( x & 1 ) odd.push_back( x );
else evn.push_back( x );
}
if( r == 2 )
{
swap( odd, odd );
odd.erase( find( odd.begin(), odd.end(), 5 ) );
odd.push_back( 5 );
}
else if( r == 3 )
{
odd.erase( odd.begin() ); odd.erase( odd.begin() );
odd.push_back( 1 ); odd.push_back( 3 );
evn.erase( evn.begin() ); evn.push_back( 2 );
}
vector<uint>::iterator it = evn.begin();
while( it != evn.end() )
{
solution.push_back( ( *it ) );
it++;
}
it = odd.begin();
while( it != odd.end() )
{
solution.push_back( ( *it ) );
it++;
}
}

vector<uint> odd, evn, solution;
};
//--------------------------------------------------------------------------------------------------
int main( int argc, char* argv[] )
{
uint n; nQueens_Heuristic nQH;
while( true )
{
cout << "Enter board size bigger than 3 (0 - 3 to QUIT): "; cin >> n;
if( n < 4 ) return 0;
nQH.solve( n ); cout << endl << endl;
}
return 0;
}
//--------------------------------------------------------------------------------------------------
```

## Clojure

This produces all solutions by essentially a backtracking algorithm. The heart is the extends? function, which takes a partial solution for the first k<size columns and sees if the solution can be extended by adding a queen at row n of column k+1. The extend function takes a list of all partial solutions for k columns and produces a list of all partial solutions for k+1 columns. The final list solutions is calculated by starting with the list of 0-column solutions (obviously this is the list [ [] ], and iterates extend for size times.

```(def size 8)

(defn extends? [v n]
(let [k (count v)]
(not-any? true?
(for [i (range k) :let [vi (v i)]]
(or
(= vi n)  ;check for shared row
(= (- k i) (Math/abs (- n vi)))))))) ;check for shared diagonal

(defn extend [vs]
(for [v vs
n (range 1 (inc size)) :when (extends? v n)]
(conj v n)))

(def solutions
(nth (iterate extend [[]]) size))

(doseq [s solutions]
(println s))

(println (count solutions) "solutions")
```

### Short Version

```(ns queens
(:require [clojure.math.combinatorics :as combo]

(defn queens [n]
(filter (fn [x] (every? #(apply distinct? (map-indexed % x)) [+ -]))
(combo/permutations (range 1 (inc n)))))
```

### Backtracking as Tree processing

Each state of the board can be represented as a sequence of the row coordinate for a queen, the column being the index in the sequence (coordinates starting at 0). Each state can have 'children' states if it is legal (no conflict) and has less than n queens. A child state is the result of adding a new queen on the next column, there are as many children states as rows as we are trying all of them. A depth first traversal of this virtual tree of states gives us the solutions when we filter out the illegal states and the incomplete states. The sequence of states is lazy so we could read only one result and not have to compute the other states.

```  (defn n-queens [n]
(let[children #(map (partial conj %) (range n))
no-conflict? (fn [x] (or (empty? x)
(every? #(apply distinct? (map-indexed % x))
[+ - (fn[_ v] v)])))]
(filter (every-pred no-conflict? #(= n (count %)))
(tree-seq (every-pred #(> n (count %))
no-conflict?)
children []))))
```

## CLU

Translation of: C
```n_queens = cluster is solve
rep = null
own hist: array[int] := array[int]\$[]
own solutions: array[string] := array[string]\$[]

attack = proc (i,j,col: int) returns (bool)
return(hist[j]=i | int\$abs(hist[j]-i)=col-j)
end attack

cur_solution = proc ()
n: int := array[int]\$size(hist)
ss: stream := stream\$create_output()

for i: int in int\$from_to(0,n-1) do
for j: int in int\$from_to(0,n-1) do
if j=hist[i] then stream\$putc(ss, 'Q')
elseif (i+j)//2 = 1 then stream\$putc(ss, ' ')
else stream\$putc(ss, '.')
end
end
stream\$putc(ss, '\n')
end

end cur_solution

solve_rec = proc (col: int)
n: int := array[int]\$size(hist)
if col=n then cur_solution() return end

for i: int in int\$from_to(0,n-1) do
j: int := 0
while j<col cand ~attack(i,j,col) do j := j+1 end
if j<col then continue end
hist[col] := i
solve_rec(col+1)
end
end solve_rec

solve = proc (n: int) returns (sequence[string])
hist := array[int]\$fill(0,n,0)
solutions := array[string]\$[]
solve_rec(0)
return(sequence[string]\$a2s(solutions))
end solve
end n_queens

start_up = proc()
N = 8

po: stream := stream\$primary_output()
solutions: sequence[string] := n_queens\$solve(N)

count: int := 0
for s: string in sequence[string]\$elements(solutions) do
count := count + 1
stream\$putl(po, "No. " || int\$unparse(count) || "\n-------\n" || s)
end
end start_up```
Output:
```No. 1
-------
Q . . .
. .Q. .
. . . .Q
. . Q .
. Q . .
. . .Q.
.Q. . .
. Q . .

No. 2
-------
Q . . .
. . Q .
. . . .Q
.Q. . .
. . . Q
. Q . .
.Q. . .
. .Q. .

No. 3
-------
Q . . .
. . .Q.
. .Q. .
. . Q .
. . . .Q
Q . . .
. . Q .
.Q. . .

No. 4
-------
Q . . .
. . .Q.
. . Q .
. . . Q
.Q. . .
. Q . .
. . .Q.
.Q. . .

No. 5
-------
.Q. . .
. Q . .
. . .Q.
. . . Q
. Q . .
Q. . . .
. . . Q
. .Q. .

No. 6
-------
.Q. . .
. .Q. .
. . . Q
Q. . . .
. Q . .
. . . Q
. . .Q.
. Q . .

No. 7
-------
.Q. . .
. .Q. .
. . . Q
. Q . .
Q . . .
. . . Q
. . .Q.
.Q. . .

No. 8
-------
.Q. . .
. . Q .
Q . . .
. . .Q.
. .Q. .
. . . Q
. Q . .
. .Q. .

No. 9
-------
.Q. . .
. . Q .
. . . .Q
.Q. . .
Q . . .
. Q . .
. . . Q
. .Q. .

No. 10
-------
.Q. . .
. . .Q.
. Q . .
. . Q .
. . . .Q
. .Q. .
Q . . .
. Q . .

No. 11
-------
.Q. . .
. . .Q.
. . Q .
. . . Q
Q . . .
. Q . .
. . .Q.
.Q. . .

No. 12
-------
.Q. . .
. . . Q
. . .Q.
Q. . . .
. Q . .
. .Q. .
. . . Q
. Q . .

No. 13
-------
. Q . .
Q. . . .
. . . Q
. .Q. .
. . . .Q
Q . . .
. .Q. .
. . Q .

No. 14
-------
. Q . .
. .Q. .
.Q. . .
. . . Q
Q . . .
. . .Q.
. .Q. .
. . Q .

No. 15
-------
. Q . .
. .Q. .
.Q. . .
. . . Q
. . .Q.
. Q . .
. . . Q
Q. . . .

No. 16
-------
. Q . .
. .Q. .
. . . Q
Q. . . .
. .Q. .
Q . . .
. . . .Q
. . Q .

No. 17
-------
. Q . .
. .Q. .
. . . .Q
. Q . .
Q . . .
. . .Q.
.Q. . .
. . Q .

No. 18
-------
. Q . .
. . Q .
.Q. . .
. .Q. .
. . . .Q
Q. . . .
. . . Q
. Q . .

No. 19
-------
. Q . .
. . Q .
.Q. . .
. . .Q.
Q . . .
. Q . .
. . . .Q
. .Q. .

No. 20
-------
. Q . .
. . Q .
.Q. . .
. . .Q.
. . Q .
Q. . . .
. . . .Q
. Q . .

No. 21
-------
. Q . .
. . Q .
. .Q. .
Q. . . .
. . . .Q
. .Q. .
. . . Q
Q . . .

No. 22
-------
. Q . .
. . Q .
. .Q. .
Q . . .
. . . .Q
. .Q. .
. . . Q
Q. . . .

No. 23
-------
. Q . .
. . Q .
. . . .Q
Q. . . .
. .Q. .
. . .Q.
. . Q .
Q . . .

No. 24
-------
. Q . .
. . Q .
. . . .Q
Q. . . .
. . Q .
. . .Q.
.Q. . .
. Q . .

No. 25
-------
. Q . .
. . Q .
. . . .Q
Q . . .
. .Q. .
Q. . . .
. . . Q
. .Q. .

No. 26
-------
. Q . .
. . .Q.
.Q. . .
. . . Q
. . Q .
Q. . . .
. .Q. .
. . Q .

No. 27
-------
. Q . .
. . .Q.
.Q. . .
. . . Q
. . .Q.
. Q . .
Q . . .
. .Q. .

No. 28
-------
. Q . .
. . . Q
. .Q. .
. . .Q.
Q . . .
. . Q .
.Q. . .
. .Q. .

No. 29
-------
. .Q. .
Q. . . .
. . Q .
. . . Q
.Q. . .
. . .Q.
. Q . .
. . Q .

No. 30
-------
. .Q. .
Q. . . .
. . Q .
. . . Q
. . .Q.
.Q. . .
. . . Q
Q . . .

No. 31
-------
. .Q. .
Q . . .
. . Q .
. . . Q
. . .Q.
Q. . . .
. Q . .
. . .Q.

No. 32
-------
. .Q. .
Q . . .
. . . Q
.Q. . .
. . .Q.
. . . Q
Q . . .
. .Q. .

No. 33
-------
. .Q. .
Q . . .
. . . Q
.Q. . .
. . .Q.
. . . Q
. . Q .
Q. . . .

No. 34
-------
. .Q. .
Q . . .
. . . Q
. .Q. .
Q . . .
. . . Q
. . .Q.
.Q. . .

No. 35
-------
. .Q. .
Q . . .
. . . .Q
. .Q. .
. . . Q
Q. . . .
. Q . .
. . Q .

No. 36
-------
. .Q. .
Q . . .
. . . .Q
. . Q .
Q . . .
.Q. . .
. . Q .
. . .Q.

No. 37
-------
. .Q. .
. . Q .
Q . . .
. .Q. .
.Q. . .
. . . Q
. Q . .
. . .Q.

No. 38
-------
. .Q. .
. . Q .
. . . .Q
Q . . .
. . . Q
Q. . . .
. Q . .
. .Q. .

No. 39
-------
. .Q. .
. . Q .
. . . .Q
.Q. . .
Q . . .
. . .Q.
. . Q .
Q . . .

No. 40
-------
. .Q. .
. . .Q.
Q . . .
. . . Q
. . Q .
Q . . .
. . .Q.
.Q. . .

No. 41
-------
. .Q. .
. . .Q.
. Q . .
. . . Q
.Q. . .
. .Q. .
Q . . .
. . Q .

No. 42
-------
. .Q. .
. . .Q.
. . Q .
Q . . .
. . .Q.
Q. . . .
. Q . .
. . . Q

No. 43
-------
. .Q. .
. . .Q.
. . Q .
.Q. . .
Q . . .
. . Q .
. . . .Q
Q . . .

No. 44
-------
. .Q. .
. . . Q
Q . . .
.Q. . .
. . .Q.
Q . . .
. . . Q
. .Q. .

No. 45
-------
. .Q. .
. . . Q
Q . . .
. .Q. .
. . . Q
Q . . .
. . .Q.
.Q. . .

No. 46
-------
. .Q. .
. . . Q
. . Q .
.Q. . .
Q . . .
. . .Q.
.Q. . .
. . Q .

No. 47
-------
. . Q .
Q. . . .
. .Q. .
. . Q .
. . . .Q
Q . . .
. . . Q
.Q. . .

No. 48
-------
. . Q .
Q. . . .
. . . .Q
. Q . .
.Q. . .
. . .Q.
. Q . .
. . Q .

No. 49
-------
. . Q .
Q. . . .
. . . .Q
. . Q .
. Q . .
. . .Q.
.Q. . .
. Q . .

No. 50
-------
. . Q .
Q . . .
. .Q. .
. . Q .
. . . .Q
.Q. . .
Q . . .
. . .Q.

No. 51
-------
. . Q .
Q . . .
. .Q. .
. . .Q.
. Q . .
. . . Q
. . .Q.
Q. . . .

No. 52
-------
. . Q .
Q . . .
. . .Q.
Q. . . .
. . . Q
. Q . .
. . . .Q
.Q. . .

No. 53
-------
. . Q .
Q . . .
. . . .Q
Q. . . .
. .Q. .
. . .Q.
. Q . .
. . Q .

No. 54
-------
. . Q .
.Q. . .
Q . . .
. . Q .
. . . .Q
Q . . .
. .Q. .
. . .Q.

No. 55
-------
. . Q .
.Q. . .
Q . . .
. . .Q.
.Q. . .
. . . Q
. . .Q.
. Q . .

No. 56
-------
. . Q .
.Q. . .
. . . .Q
. Q . .
. . . Q
Q. . . .
. . .Q.
Q . . .

No. 57
-------
. . Q .
. . .Q.
Q . . .
.Q. . .
. . . .Q
. . Q .
. .Q. .
Q . . .

No. 58
-------
. . Q .
. . .Q.
Q . . .
. Q . .
.Q. . .
. . . Q
. . .Q.
.Q. . .

No. 59
-------
. . Q .
. . .Q.
.Q. . .
. Q . .
. . . .Q
Q. . . .
. Q . .
. . Q .

No. 60
-------
. . Q .
. . .Q.
.Q. . .
. . Q .
. Q . .
Q. . . .
. .Q. .
. . . Q

No. 61
-------
. . Q .
. . .Q.
.Q. . .
. . Q .
. Q . .
Q. . . .
. . . .Q
. Q . .

No. 62
-------
. . Q .
. . .Q.
. .Q. .
Q. . . .
. Q . .
. . . Q
. . .Q.
Q . . .

No. 63
-------
. . Q .
. . . Q
. .Q. .
Q. . . .
. Q . .
. . Q .
.Q. . .
. . .Q.

No. 64
-------
. . Q .
. . . Q
. .Q. .
Q. . . .
. . . Q
Q . . .
. . .Q.
.Q. . .

No. 65
-------
. . .Q.
Q. . . .
. . Q .
Q . . .
. . . .Q
.Q. . .
. . . Q
. Q . .

No. 66
-------
. . .Q.
Q . . .
. . . Q
Q. . . .
. Q . .
. .Q. .
. . . .Q
. Q . .

No. 67
-------
. . .Q.
Q . . .
. . . Q
Q. . . .
. .Q. .
. . . Q
. . Q .
.Q. . .

No. 68
-------
. . .Q.
.Q. . .
Q . . .
. . .Q.
. . Q .
. . . Q
.Q. . .
. Q . .

No. 69
-------
. . .Q.
.Q. . .
Q . . .
. . . Q
. .Q. .
Q . . .
. . . Q
. .Q. .

No. 70
-------
. . .Q.
.Q. . .
Q . . .
. . . Q
. . Q .
Q . . .
. .Q. .
. . .Q.

No. 71
-------
. . .Q.
.Q. . .
. . Q .
. . .Q.
Q . . .
. Q . .
.Q. . .
. . . Q

No. 72
-------
. . .Q.
.Q. . .
. . Q .
. . . Q
Q . . .
. Q . .
.Q. . .
. . .Q.

No. 73
-------
. . .Q.
.Q. . .
. . . Q
Q . . .
. .Q. .
. . . Q
Q . . .
. .Q. .

No. 74
-------
. . .Q.
.Q. . .
. . . Q
Q . . .
. . . .Q
. .Q. .
Q . . .
. Q . .

No. 75
-------
. . .Q.
.Q. . .
. . . Q
. Q . .
Q . . .
. . . Q
.Q. . .
. .Q. .

No. 76
-------
. . .Q.
. Q . .
Q . . .
. .Q. .
. . . .Q
Q . . .
. . . Q
.Q. . .

No. 77
-------
. . .Q.
. Q . .
.Q. . .
. . . Q
. . Q .
. . .Q.
Q . . .
.Q. . .

No. 78
-------
. . .Q.
. Q . .
. . . Q
Q. . . .
. Q . .
. .Q. .
.Q. . .
. . . Q

No. 79
-------
. . .Q.
. Q . .
. . . Q
Q. . . .
. . . .Q
Q . . .
. . Q .
.Q. . .

No. 80
-------
. . .Q.
. . . Q
.Q. . .
. Q . .
Q . . .
. . .Q.
. . Q .
.Q. . .

No. 81
-------
. . . Q
Q. . . .
. Q . .
. . . Q
. . .Q.
. Q . .
.Q. . .
. .Q. .

No. 82
-------
. . . Q
Q . . .
. .Q. .
Q. . . .
. . . .Q
. .Q. .
. Q . .
. . Q .

No. 83
-------
. . . Q
Q . . .
. . .Q.
.Q. . .
Q . . .
. Q . .
. . . .Q
. .Q. .

No. 84
-------
. . . Q
.Q. . .
Q . . .
. . Q .
. . . .Q
. .Q. .
.Q. . .
. Q . .

No. 85
-------
. . . Q
.Q. . .
. . . .Q
Q . . .
. . Q .
Q. . . .
. . .Q.
. Q . .

No. 86
-------
. . . Q
. Q . .
.Q. . .
. .Q. .
. . . .Q
Q. . . .
. Q . .
. . Q .

No. 87
-------
. . . Q
. Q . .
.Q. . .
. . . Q
. . .Q.
Q. . . .
. Q . .
. .Q. .

No. 88
-------
. . . Q
. .Q. .
. Q . .
Q. . . .
. . .Q.
. . . Q
.Q. . .
. Q . .

No. 89
-------
. . . .Q
Q . . .
. .Q. .
Q. . . .
. . . Q
. .Q. .
. Q . .
. . Q .

No. 90
-------
. . . .Q
Q . . .
. . Q .
.Q. . .
Q . . .
. . .Q.
. .Q. .
. . Q .

No. 91
-------
. . . .Q
.Q. . .
Q . . .
. . Q .
.Q. . .
. .Q. .
. . . Q
. Q . .

No. 92
-------
. . . .Q
. Q . .
Q . . .
.Q. . .
. . .Q.
Q . . .
. . . Q
. .Q. .```

## CoffeeScript

```# Unlike traditional N-Queens solutions that use recursion, this
# program attempts to more closely model the "human" algorithm.
#
# In this algorithm, the function keeps placing queens on the board
# until there is no longer a safe square.  If the 8th queen has been
# placed, the solution is noted.  If fewer than 8th queens have been
# placed, then you are at a dead end.  In either case, backtracking occurs.
# The LAST queen placed on the board gets pulled, then it gets moved
# to the next safe square.  (We backtrack even after a "good" attempt in
# order to get to a new solution.)  This backtracking may repeat itself
# several times until the original misplaced queen finally is proven to
#
# Many N-Queens solutions use lazy logic (along with geometry shortcuts)
# to determine whether a queen is under attack.  In this algorithm, we
# are more proactive, essentially updating a sieve every time we lay a
# queen down.  To make backtracking easier, the sieve uses ref-counts vs.
# a simple safe/unsafe boolean.
#
# We precompute the "attack graph" up front, and then we essentially ignore
# the geometry of the problem.  This approach, while perhaps suboptimal for
# queens, probably is more flexible for general "coexistence" problems.
nqueens = (n) ->
neighbors = precompute_neighbors(n)

board = []
num_solutions = 0
num_backtracks = 0
queens = []
pos = 0

for p in [0...n*n]
board.push 0

attack = (pos, delta=1) ->
for neighbor in neighbors[pos]
board[neighbor] += delta

backtrack = ->
pos = queens.pop()
attack pos, -1 # unattack queen you just pulled
pos += 1
num_backtracks += 1

# The following loop finds all 92 solutions to
# the 8-queens problem (for n=8).
while true
if pos >= n*n
if queens.length == 0
break
backtrack()
continue

# If a square is empty
if board[pos] == 0
attack pos
queens.push pos
if queens.length == n
num_solutions += 1
show_queens queens, n
backtrack()
pos += 1

console.log "#{num_solutions} solutions"
console.log "#{num_backtracks} backtracks"

precompute_neighbors = (n) ->
# For each board position, build a list of all
# the board positions that would be under attack if
# you placed a queen on it.  This assumes a 1d array
# of squares.
neighbors = []

find_neighbors = (pos) ->
arr = []
row = Math.floor pos / n
col = pos % n
for i in [0...n]
if i != col
arr.push row*n + i
r1 = row + col - i
r2 = row + i - col
if 0 <= r1 and r1 < n
arr.push r1*n + i
if 0 <= r2 and r2 < n
arr.push r2*n + i
if i != row
arr.push i*n + col
arr

for pos in [0...n*n]
neighbors.push find_neighbors(pos)
neighbors

show_queens = (queens, n) ->
# precondition: queens is a sorted array of integers,
# and each row is represented
console.log "\n------"
for q in queens
col = q % n
s = ''
for c in [0...n]
if c == col
s += "Q "
else
s += "* "
console.log s + "\n"

nqueens(8)
```

## Common Lisp

```(defun queens (n &optional (m n))
(if (zerop n)
(list nil)
(loop for solution in (queens (1- n) m)
nconc (loop for new-col from 1 to m
when (loop for row from 1 to n
for col in solution
always (/= new-col col (+ col row) (- col row)))
collect (cons new-col solution)))))

(defun print-solution (solution)
(loop for queen-col in solution
do (loop for col from 1 to (length solution)
do (write-char (if (= col queen-col) #\Q #\.)))
(terpri))
(terpri))

(defun print-queens (n)
(mapc #'print-solution (queens n)))
```

### Alternate solution

Translation of Fortran 77

```(defun queens1 (n)
(let ((a (make-array n))
(s (make-array n))
(u (make-array (list (- (* 4 n) 2)) :initial-element t))
y z (i 0) j p q (r (1- (* 2 n))) (m 0))
(dotimes (i n) (setf (aref a i) i))
(tagbody
L1
(if (>= i n) (go L5))
(setf j i)
L2
(setf y (aref a j) z (aref a i))
(setf p (+ (- i y) n -1) q (+ i y))
(setf (aref a i) y (aref a j) z)
(when (and (aref u p) (aref u (+ q r)))
(setf (aref s i) j (aref u p) nil (aref u (+ q r)) nil)
(incf i)
(go L1))
L3
(incf j)
(if (< j n) (go L2))
L4
(decf j)
(if (= j i) (go L6))
(rotatef (aref a i) (aref a j))
(go L4)
L5
(incf m)
L6
(decf i)
(if (minusp i) (go L7))
(setf p (+ (- i (aref a i)) n -1) q (+ i (aref a i)) j (aref s i))
(setf (aref u p) t (aref u (+ q r)) t)
(go L3)
L7)
m))

> (loop for n from 1 to 14 collect (cons n (queens1 n)))
((1 . 1) (2 . 0) (3 . 0) (4 . 2) (5 . 10) (6 . 4) (7 . 40) (8 . 92) (9 . 352)
(10 . 724) (11 . 2680) (12 . 14200) (13 . 73712) (14 . 365596))
```

As in Fortran, the iterative function above is equivalent to the recursive function below:

```(defun queens2 (n)
(let ((a (make-array n))
(u (make-array (+ n n -1) :initial-element t))
(v (make-array (+ n n -1) :initial-element t))
(m 0))
(dotimes (i n) (setf (aref a i) i))
(labels ((sub (i)
(if (= i n)
;(push (copy-seq a) s)
(incf m)
(loop for k from i below n do
(let ((p (+ i (aref a k)))
(q (+ (- i (aref a k)) n -1)))
(when (and (aref u p) (aref v q))
(setf (aref u p) nil (aref v q) nil)
(rotatef (aref a i) (aref a k))
(sub (1+ i))
(setf (aref u p) t (aref v q) t)
(rotatef (aref a i) (aref a k))))))))
(sub 0))
m))
```

## Curry

Three different ways of attacking the same problem. All copied from A Catalog of Design Patterns in FLP

```-- 8-queens implementation with the Constrained Constructor pattern
-- Sergio Antoy
-- Fri Jul 13 07:05:32 PDT 2001

-- Place 8 queens on a chessboard so that no queen can capture
-- (and be captured by) any other queen.

-- Non-deterministic choice operator

infixl 0 !
X ! _ = X
_ ! Y = Y

-- A solution is represented by a list of integers.
-- The i-th integer in the list is the column of the board
-- in which the queen in the i-th row is placed.
-- Rows and columns are numbered from 1 to 8.
-- For example, [4,2,7,3,6,8,5,1] is a solution where the
-- the queen in row 1 is in column 4, etc.
-- Any solution must be a permutation of [1,2,...,8].

-- The state of a queen is its position, row and column, on the board.
-- Operation column is a particularly simple instance
-- of a Constrained Constructor pattern.
-- When it is invoked, it produces only valid states.

column = 1 ! 2 ! 3 ! 4 ! 5 ! 6 ! 7 ! 8

-- A path of the puzzle is a sequence of successive placements of
-- queens on the board.  It is not explicitly defined as a type.
-- A path is a potential solution in the making.

-- Constrained Constructor on a path
-- Any path must be valid, i.e., any column must be in the range 1..8
-- and different from any other column in the path.
-- Furthermore, the path must be safe for the queens.
-- No queen in a path may capture any other queen in the path.
-- Operation makePath add column n to path c or fails.

makePath c n | valid c && safe c 1 = n:c
where valid c | n =:= column = uniq c
where uniq [] = True
uniq (c:cs) = n /= c && uniq cs
safe [] _ = True
safe (c:cs) k = abs (n-c) /= k && safe cs (k+1)
where abs x = if x < 0 then -x else x

-- extend the path argument till all the queens are on the board
-- see the Incremental Solution pattern

extend p = if (length p == 8)
then p
else extend (makePath p x)
where x free

-- solve the puzzle

main = extend []```

Another approach from the same source.

```-- N-queens puzzle implemented with "Distinct Choices" pattern
-- Sergio Antoy
-- Tue Sep  4 13:16:20 PDT 2001
-- updated: Mon Sep 23 15:22:15 PDT 2002

import Integer

queens x | y =:= permute x & void (capture y) = y  where y free

capture y = let l1,l2,l3,y1,y2 free in
l1 ++ [y1] ++ l2 ++ [y2] ++ l3 =:= y & abs (y1-y2) =:= length l2 + 1

-- negation as failure (implemented by encapsulated search):
void c = (findall \_->c) =:= []

-- How does this permutation algorithm work?
-- Only the elements [0,1,...,n-1] can be permuted.
-- The reason is that each element is used as an index in a list.
-- A list, called store, of free variables of length n is created.
-- Then, the n iterations described below are executed.
-- At the i-th iteration, an element, say s,
-- of the initial list is non-deterministically selected.
-- This element is used as index in the store.
-- The s-th variable of the store is unified with i.
-- At the end of the iterations, the elements of the store
-- are a permutation of [0,1,...,n-1], i.e., the elements
-- are unique since two iterations cannot select the same index.

permute n = result n
where result n = if n==0 then [] else pick n store : result (n-1)
pick i store | store !! k =:= i = k where k = range n
range n | n > 0 = range (n-1) ! (n-1)
store = free
-- end```

Yet another approach, also from the same source.

```-- 8-queens implementation with both the Constrained Constructor
-- and the Fused Generate and Test patterns.
-- Sergio Antoy
-- Fri Jul 13 07:05:32 PDT 2001

-- Place 8 queens on a chessboard so that no queen can capture
-- (and be captured by) any other queen.

-- Non-deterministic choice operator

infixl 0 !
X ! _ = X
_ ! Y = Y

-- A solution is represented by a list of integers.
-- The i-th integer in the list is the column of the board
-- in which the queen in the i-th row is placed.
-- Rows and columns are numbered from 1 to 8.
-- For example, [4,2,7,3,6,8,5,1] is a solution where the
-- the queen in row 1 is in column 4, etc.
-- Any solution must be a permutation of [1,2,...,8].

-- The state of a queen is its position, row and column, on the board.
-- Operation column is a particularly simple instance
-- of a Constrained Constructor pattern.
-- When it is invoked, it produces only valid states.

column = 1 ! 2 ! 3 ! 4 ! 5 ! 6 ! 7 ! 8

-- A path of the puzzle is a sequence of successive placements of
-- queens on the board.  It is not explicitly defined as a type.
-- A path is a potential solution in the making.

-- Constrained Constructor on a path
-- Any path must be valid, i.e., any column must be in the range 1..8
-- and different from any other column in the path.
-- Furthermore, the path must be safe for the queens.
-- No queen in a path may capture any other queen in the path.
-- Operation makePath add column n to path c or fails.

makePath c n | valid c && safe c 1 = n:c
where valid c | n =:= column = uniq c
where uniq [] = True
uniq (c:cs) = n /= c && uniq cs
safe [] _ = True
safe (c:cs) k = abs (n-c) /= k && safe cs (k+1)
where abs x = if x < 0 then -x else x

-- extend the path argument till all the queens are on the board
-- see the Incremental Solution pattern

extend p = if (length p == 8)
then p
else extend (makePath p x)
where x free

-- solve the puzzle

main = extend []```

Mainly webpakcs, uses constraint-solver.

```import CLPFD
import Findall

queens n qs =
qs =:= [_ | _ <- [1..n]]
& domain qs 1 (length qs)
& allDifferent qs
& allSafe qs
& labeling [FirstFail] qs

allSafe [] = success
allSafe (q:qs) = safe q qs 1 & allSafe qs

safe :: Int -> [Int] -> Int -> Success
safe _     []  _ = success
safe q (q1:qs) p = q /=# q1+#p & q /=# q1-#p & safe q qs (p+#1)

-- oneSolution  = unpack  \$ queens 8
-- allSolutions = findall \$ queens 8```

## D

### Short Version

This high-level version uses the second solution of the Permutations task.

```void main() {
import std.stdio, std.algorithm, std.range, permutations2;

enum n = 8;
n.iota.array.permutations.filter!(p =>
n.iota.map!(i => p[i] + i).array.sort().uniq.count == n &&
n.iota.map!(i => p[i] - i).array.sort().uniq.count == n)
.count.writeln;
}
```
Output:
`92`

### Intermediate Version

This version shows all the solutions.

Translation of: C
```enum side = 8;
__gshared int[side] board;

bool isUnsafe(in int y) nothrow @nogc {
immutable int x = board[y];
foreach (immutable i; 1 .. y + 1) {
immutable int t = board[y - i];
if (t == x || t == x - i || t == x + i)
return true;
}

return false;
}

void showBoard() nothrow @nogc {
import core.stdc.stdio;

static int s = 1;
printf("\nSolution #%d:\n", s++);
foreach (immutable y; 0 .. side) {
foreach (immutable x; 0 .. side)
putchar(board[y] == x ? 'Q' : '.');
putchar('\n');
}
}

void main() nothrow @nogc {
int y = 0;
board = -1;

while (y >= 0) {
do {
board[y]++;
} while (board[y] < side && y.isUnsafe);

if (board[y] < side) {
if (y < (side - 1))
board[++y] = -1;
else
showBoard;
} else
y--;
}
}
```
Output:
```Solution #1:
Q.......
....Q...
.......Q
.....Q..
..Q.....
......Q.
.Q......
...Q....

[...]

Solution #91:
.......Q
..Q.....
Q.......
.....Q..
.Q......
....Q...
......Q.
...Q....

Solution #92:
.......Q
...Q....
Q.......
..Q.....
.....Q..
.Q......
......Q.
....Q...```

### Fast Version

Translation of: C
```ulong nQueens(in uint nn) pure nothrow @nogc @safe
in {
assert(nn > 0 && nn <= 27,
"'side' value must be in 1 .. 27.");
} body {
if (nn < 4)
return nn == 1;

enum uint ulen = uint.sizeof * 8;
immutable uint full = uint.max - ((1 << (ulen - nn)) - 1);
immutable n = nn - 3;

typeof(return) count;
uint l=void, r=void, c=void;
uint mm; // mm and mmi are a stack.

// Require second queen to be left of the first queen, so
// we ever only test half of the possible solutions. This
// is why we can't handle n=1 here.
for (uint b0 = 1U << (ulen - n - 3); b0; b0 <<= 1) {
for (uint b1 = b0 << 2; b1; b1 <<= 1) {
uint d = n;
// c: columns occupied by previous queens.
c[n] = b0 | b1;
// l: columns attacked by left diagonals.
l[n] = (b0 << 2) | (b1 << 1);
// r: by right diagnoals.
r[n] = (b0 >> 2) | (b1 >> 1);

// Availabe columns on current row.
uint bits = full & ~(l[n] | r[n] | c[n]);

uint mmi = 1;
mm[mmi] = bits;

while (bits) {
// d: depth, aka row. counting backwards.
// Because !d is often faster than d != n.
while (d) {
// immutable uint pos = 1U << bits.bsf; // Slower.
immutable uint pos = -int(bits) & bits;

// Mark bit used. Only put current bits on
// stack if not zero, so backtracking will
// skip exhausted rows (because reading stack
// variable is slow compared to registers).
bits &= ~pos;
if (bits) {
mm[mmi] = bits | d;
mmi++;
}

d--;
l[d] = (l[d + 1] | pos) << 1;
r[d] = (r[d + 1] | pos) >> 1;
c[d] =  c[d + 1] | pos;

bits = full & ~(l[d] | r[d] | c[d]);

if (!bits)
break;
if (!d) {
count++;
break;
}
}

// Bottom of stack m is a zero'd field acting as
// sentinel.  When saving to stack, left 27 bits
// are the available columns, while right 5 bits
// is the depth. Hence solution is limited to size
// 27 board -- not that it matters in foreseeable
// future.
mmi--;
bits = mm[mmi];
d = bits & 31U;
bits &= ~31U;
}
}
}

return count * 2;
}

void main(in string[] args) {
import std.stdio, std.conv;

immutable uint side = (args.length >= 2) ? args.to!uint : 8;
writefln("N-queens(%d) = %d solutions.", side, side.nQueens);
}
```
Output:
`N-queens(8) = 92 solutions.`

With side = 17:

`N-queens(17) = 95815104 solutions.`

Run-time for side = 17 compiled with ldc2 is about 49.5 seconds.

`N-queens(19) = 4968057848 solutions.`

## Dart

```/**
Return true if queen placement q[n] does not conflict with
other queens q through q[n-1]
*/
isConsistent(List q, int n) {
for (int i=0; i<n; i++) {
if (q[i] == q[n]) {
return false; // Same column
}

if ((q[i] - q[n]) == (n - i)) {
return false; // Same major diagonal
}

if ((q[n] - q[i]) == (n - i)) {
return false; // Same minor diagonal
}
}

return true;
}

/**
Print out N-by-N placement of queens from permutation q in ASCII.
*/
printQueens(List q) {
int N = q.length;
for (int i=0; i<N; i++) {
StringBuffer sb = new StringBuffer();
for (int j=0; j<N; j++) {
if (q[i] == j) {
sb.write("Q ");
} else {
sb.write("* ");
}
}
print(sb.toString());
}
print("");
}

/**
Try all permutations using backtracking
*/
enumerate(int N) {
var a = new List(N);
_enumerate(a, 0);
}

_enumerate(List q, int n) {
if (n == q.length) {
printQueens(q);
} else {
for (int i = 0; i < q.length; i++) {
q[n] = i;
if (isConsistent(q, n)){
_enumerate(q, n+1);
}
}
}
}

void main() {
enumerate(4);
}
```
Output:
```* Q * *
* * * Q
Q * * *
* * Q *

* * Q *
Q * * *
* * * Q
* Q * *
```

## Delphi

Translation of: Go
```program N_queens_problem;

{\$APPTYPE CONSOLE}

uses
System.SysUtils;

var
i: Integer;
q: boolean;
a: array[0..8] of boolean;
b: array[0..16] of boolean;
c: array[0..14] of boolean;
x: array[0..8] of Integer;

procedure TryMove(i: Integer);
begin
var j := 1;
while True do
begin
q := false;
if a[j] and b[i + j] and c[i - j + 7] then
begin
x[i] := j;
a[j] := false;
b[i + j] := false;
c[i - j + 7] := false;

if i < 8 then
begin
TryMove(i + 1);
if not q then
begin
a[j] := true;
b[i + j] := true;
c[i - j + 7] := true;
end;
end
else
q := true;
end;
if q or (j = 8) then
Break;
inc(j);
end;
end;

begin
for i := 1 to 8 do
a[i] := true;

for i := 2 to 16 do
b[i] := true;

for i := 0 to 14 do
c[i] := true;

TryMove(1);

if q then
for i := 1 to 8 do
writeln(i, ' ', x[i]);
end.
```

## Draco

Translation of: C
```byte SIZE = 8;
word count;

proc solve([*] int hist; int col) void:
int i, j, n;
n := dim(hist, 1);
if col = n then
count := count + 1;
writeln();
writeln("No. ", count);
writeln("-----");
for i from 0 upto n-1 do
for j from 0 upto n-1 do
write(if j=hist[i] then 'Q'
elif (i+j)&1 /= 0 then ' '
else '.' fi)
od;
writeln()
od
else
for i from 0 upto n-1 do
j := 0;
while j<col and not (hist[j]=i or |(hist[j]-i) = col-j) do
j := j + 1
od;
if j >= col then
hist[col] := i;
solve(hist, col+1)
fi
od
fi
corp

proc nonrec main() void:
[SIZE] int hist;
count := 0;
solve(hist, 0)
corp```
Output:
```No. 1
-----
Q . . .
. .Q. .
. . . .Q
. . Q .
. Q . .
. . .Q.
.Q. . .
. Q . .```

...

```No. 92
-----
. . . .Q
. Q . .
Q . . .
.Q. . .
. . .Q.
Q . . .
. . . Q
. .Q. .```

## EasyLang

```subr show_sol
print "Solution " & n_sol
print ""
for i = 1 to n
write "  "
for j = 1 to n
if j = x[i]
write "Q "
else
write ". "
.
.
print ""
.
print ""
.
subr test
ok = 1
for i = 1 to y - 1
if x[y] = x[i] or abs (x[i] - x[y]) = abs (y - i)
ok = 0
.
.
.
n = 8
len x[] n
y = 1
x = 1
while y >= 1
test
if ok = 1 and y + 1 <= n
y += 1
x[y] = 1
else
if ok = 1
n_sol += 1
if n_sol <= 1
show_sol
.
.
while y >= 1 and x[y] = n
y -= 1
.
if y >= 1
x[y] += 1
.
.
.
print n_sol & " solutions"```
Output:
```Solution 1

Q . . . . . . .
. . . . Q . . .
. . . . . . . Q
. . . . . Q . .
. . Q . . . . .
. . . . . . Q .
. Q . . . . . .
. . . Q . . . .

92 solutions```

## EchoLisp

```;; square num is i + j*N
(define-syntax-rule (sq i j) (+ i (* j N)))

;; compute diag number for each square
(define (do-diag1 i0 j0  dnum  into: dnum1 N) ;; ++i and ++j diags
(for [(i (in-range i0 N)) (j (in-range j0 N))]
;;(writeln   i j 'diag1 dnum)
(vector-set! dnum1 (sq i j) dnum)))

(define (do-diag2 i0 j0  dnum into: dnum2 N) ;; --i and ++j diags
(for [(i (in-range i0 -1 -1)) (j (in-range j0 N))]
;; (writeln i j 'diag2 dnum)
(vector-set! dnum2 (sq i j) dnum)))

(define (init-diags dnum1 dnum2 N)
(define dnum 0)
(for ((j N)) (do-diag1 0 j dnum dnum1 N) (++ dnum))
(for ((i (in-range 1 N)))
(do-diag1 i 0 dnum dnum1  N) (++ dnum))
(set! dnum 0)
(for ((j N)) (do-diag2 (1- N) j dnum dnum2 N) (++ dnum))
(for ((i (1- N))) (do-diag2 i 0 dnum dnum2 N) (++ dnum)))
;; end boring diags part

(define (q-search i  N col diag1 diag2 dnum1 dnum2    &hits (ns))
(cond
[(= i N)  (set-box! &hits (1+ (unbox &hits))) ] ;;  (writeln  'HIT col)
[else

(for ((j N))
(set! ns (sq i j))
#:continue (or [col j] [diag1 [dnum1 ns]] [diag2 [dnum2 ns]])
(vector-set! col j i) ;; move
(vector-set! diag1 [dnum1 ns] #t) ;; flag busy diagonal
(vector-set! diag2 [dnum2 ns] #t)
(q-search (1+ i) N col diag1 diag2 dnum1 dnum2 &hits)
(vector-set! col j #f) ;; unmove
(vector-set! diag1 [dnum1 ns] #f)
(vector-set! diag2 [dnum2 ns] #f))
]))

(define (q-count (N 8))
(define dnum1 (make-vector (* N N)))
(define dnum2 (make-vector (* N N )))
(init-diags dnum1 dnum2 N)

(define diag1 (make-vector (* 2 N) #f)) ; busy diag's
(define diag2 (make-vector (* 2 N) #f))
(define col (make-vector N  #f))
(define &hits (box 0))

(q-search 0 N col diag1 diag2 dnum1 dnum2  &hits)
(unbox &hits))

(for ((i up-to-n)) (writeln i ' ♕ (q-count i) 'solutions)))
```
Output:
```(task 13)

0     ♕     1     solutions
1     ♕     1     solutions
2     ♕     0     solutions
3     ♕     0     solutions
4     ♕     2     solutions
5     ♕     10     solutions
6     ♕     4     solutions
7     ♕     40     solutions
8     ♕     92     solutions
9     ♕     352     solutions
10     ♕     724     solutions
11     ♕     2680     solutions
12     ♕     14200     solutions
```

## Eiffel

```class
QUEENS

create
make

feature {NONE}
counter: INTEGER

place_queens(board: ARRAY[INTEGER]; level: INTEGER)
local
i, j: INTEGER
safe: BOOLEAN
do
if level > board.count
then
counter := counter + 1
else
from
i := 1
until
i > board.count
loop
safe := True
from
j := 1
until
j = level or not safe
loop
if (board[j] = i)
or (j - level = i - board[j])
or (j - level = board[j] - i)
then
safe := False
end
j := j + 1
end
if safe
then
board[level] := i
place_queens(board, level + 1)
end
i := i + 1
end
end
end

feature
possible_positions_of_n_queens(n: INTEGER): INTEGER
local
board: ARRAY[INTEGER]
do
create board.make_filled (0, 1, n)
counter := 0
place_queens(board, 1)
Result := counter
end

make
local
n: INTEGER
do
io.put_string ("Please enter the number of queens: ")
n := io.last_integer
print("%NPossible number of placings: " + possible_positions_of_n_queens(n).out + "%N")
end
end
```
Output:
```Please enter the number of queens: 1
Possible number of placings: 1

Please enter the number of queens: 2
Possible number of placings: 0

Please enter the number of queens: 3
Possible number of placings: 0

Please enter the number of queens: 4
Possible number of placings: 2

Please enter the number of queens: 5
Possible number of placings: 10

Please enter the number of queens: 6
Possible number of placings: 4

Please enter the number of queens: 7
Possible number of placings: 40

Please enter the number of queens: 8
Possible number of placings: 92

Please enter the number of queens: 9
Possible number of placings: 352

Please enter the number of queens: 10
Possible number of placings: 724
```

## Elixir

Translation of: Ruby
```defmodule RC do
def queen(n, display \\ true) do
solve(n, [], [], [], display)
end

defp solve(n, row, _, _, display) when n==length(row) do
if display, do: print(n,row)
1
end
defp solve(n, row, add_list, sub_list, display) do
Enum.map(Enum.to_list(0..n-1) -- row, fn x ->
add = x + length(row)             # \ diagonal check
sub = x - length(row)             # / diagonal check
0
else
end
end) |> Enum.sum                    # total of the solution
end

defp print(n, row) do
IO.puts frame = "+" <> String.duplicate("-", 2*n+1) <> "+"
Enum.each(row, fn x ->
line = Enum.map_join(0..n-1, fn i -> if x==i, do: "Q ", else: ". " end)
IO.puts "| #{line}|"
end)
IO.puts frame
end
end

Enum.each(1..6, fn n ->
IO.puts " #{n} Queen : #{RC.queen(n)}"
end)

Enum.each(7..12, fn n ->
IO.puts " #{n} Queen : #{RC.queen(n, false)}"             # no display
end)
```
Output:
```+---+
| Q |
+---+
1 Queen : 1
2 Queen : 0
3 Queen : 0
+---------+
| . . Q . |
| Q . . . |
| . . . Q |
| . Q . . |
+---------+
+---------+
| . Q . . |
| . . . Q |
| Q . . . |
| . . Q . |
+---------+
4 Queen : 2
+-----------+
| . . . Q . |
| . Q . . . |
| . . . . Q |
| . . Q . . |
| Q . . . . |
+-----------+
+-----------+
| . . Q . . |
| . . . . Q |
| . Q . . . |
| . . . Q . |
| Q . . . . |
+-----------+
+-----------+
| . . . . Q |
| . . Q . . |
| Q . . . . |
| . . . Q . |
| . Q . . . |
+-----------+
+-----------+
| . . . Q . |
| Q . . . . |
| . . Q . . |
| . . . . Q |
| . Q . . . |
+-----------+
+-----------+
| . . . . Q |
| . Q . . . |
| . . . Q . |
| Q . . . . |
| . . Q . . |
+-----------+
+-----------+
| Q . . . . |
| . . . Q . |
| . Q . . . |
| . . . . Q |
| . . Q . . |
+-----------+
+-----------+
| . Q . . . |
| . . . . Q |
| . . Q . . |
| Q . . . . |
| . . . Q . |
+-----------+
+-----------+
| Q . . . . |
| . . Q . . |
| . . . . Q |
| . Q . . . |
| . . . Q . |
+-----------+
+-----------+
| . . Q . . |
| Q . . . . |
| . . . Q . |
| . Q . . . |
| . . . . Q |
+-----------+
+-----------+
| . Q . . . |
| . . . Q . |
| Q . . . . |
| . . Q . . |
| . . . . Q |
+-----------+
5 Queen : 10
+-------------+
| . . . . Q . |
| . . Q . . . |
| Q . . . . . |
| . . . . . Q |
| . . . Q . . |
| . Q . . . . |
+-------------+
+-------------+
| . . . Q . . |
| Q . . . . . |
| . . . . Q . |
| . Q . . . . |
| . . . . . Q |
| . . Q . . . |
+-------------+
+-------------+
| . . Q . . . |
| . . . . . Q |
| . Q . . . . |
| . . . . Q . |
| Q . . . . . |
| . . . Q . . |
+-------------+
+-------------+
| . Q . . . . |
| . . . Q . . |
| . . . . . Q |
| Q . . . . . |
| . . Q . . . |
| . . . . Q . |
+-------------+
6 Queen : 4
7 Queen : 40
8 Queen : 92
9 Queen : 352
10 Queen : 724
11 Queen : 2680
12 Queen : 14200
```

## Emacs Lisp

```(let ((*result* '()))
(defun grid-cnt (n)
(* n n) )
(defun x-axis (n pos)
(/ pos n) )
(defun y-axis (n pos)
(% pos n) )
(defun chess-cnt (chess-map)
(seq-count (lambda (x) x) chess-map))
(defun check-conflict (n chess-map pos)
(let ((is-conflict nil))
(cl-loop for i from 0 to (1- (grid-cnt n)) while (not is-conflict) do
(when (aref chess-map i)
(when (or (= (x-axis n i) (x-axis n pos))
(= (y-axis n i) (y-axis n pos))
(= (abs (- (x-axis n i) (x-axis n pos)))
(abs (- (y-axis n i) (y-axis n pos))))
)
(setq is-conflict 't)
)
)
)
is-conflict )
)

(defun place-chess (n chess-map start-pos)
(if (< (chess-cnt chess-map) n)
(progn
(let ()
(cl-loop for i from start-pos to (1- (grid-cnt n)) do
(when (not (aref chess-map i)) ;; check if place is empty
;; check if place is on hold by other chess
(when (not (check-conflict n chess-map i))
(let ((map1 (copy-sequence chess-map)))
(aset map1 i 't)
(place-chess n map1 i)
)
)
)
)
)
)
(progn
(if *result* (nconc *result* (list chess-map)) (setq *result* (list chess-map)))
)
)
)

(defun show-result (n)
(let ()
(seq-map (lambda (map1)

(let ((map-txt ""))
(message ">>>>>>>>>>>>>>")
(seq-map-indexed (lambda (elm idx)
(if (= (% idx n) 0)
;;(setq map-text (concat map-txt "\n"))
(progn
(message map-txt)
(setq map-txt "") )
)
(setq map-txt
(concat map-txt (if elm "✓" "⓪")))
) map1)
(message "<<<<<<<<<<<<<<\n")
)
) *result*)
)
(message "%d solutions in total" (length *result*))
)

(defun start-calculate (n)
(let ((chess-map (make-vector (grid-cnt n) nil)))
(place-chess n chess-map 0)
)
(show-result n)
)

(start-calculate 8)
)
```
Output:
```...
92 solutions in total
```

## Erlang

Instead of spawning a new process to search for each possible solution I backtrack.

```-module( n_queens ).

display( Board ) ->
%% Queens are in the positions in the Board list.
%% Top left corner is {1, 1}, Bottom right is {N, N}. There is a queen in the max column.
N = lists:max( [X || {X, _Y} <- Board] ),
[display_row(Y, N, Board) || Y <- lists:seq(1, N)].

solve( N ) ->
Positions = [{X, Y} || X <- lists:seq(1, N), Y <- lists:seq(1, N)],
try
bt( N, Positions, [] )

catch
_:{ok, Board} -> Board

end.

bt( N, Positions, Board ) -> bt_reject( is_not_allowed_queen_placement(N, Board), N, Positions, Board ).

bt_accept( true, _N, _Positions, Board ) -> erlang:throw( {ok, Board} );
bt_accept( false, N, Positions, Board ) -> bt_loop( N, Positions, [], Board ).

bt_loop( _N, [], _Rejects, _Board ) -> failed;
bt_loop( N, [Position | T], Rejects, Board ) ->
bt( N, T ++ Rejects, [Position | Board] ),
bt_loop( N, T, [Position | Rejects], Board ).

bt_reject( true, _N, _Positions, _Board ) -> backtrack;
bt_reject( false, N, Positions, Board ) -> bt_accept( is_all_queens(N, Board), N, Positions, Board ).

diagonals( N, {X, Y} ) ->
D1 = diagonals( N, X + 1, fun diagonals_add1/1, Y + 1, fun diagonals_add1/1 ),
D2 = diagonals( N, X + 1, fun diagonals_add1/1, Y - 1, fun diagonals_subtract1/1 ),
D3 = diagonals( N, X - 1, fun diagonals_subtract1/1, Y + 1, fun diagonals_add1/1 ),
D4 = diagonals( N, X - 1, fun diagonals_subtract1/1, Y - 1, fun diagonals_subtract1/1 ),
D1 ++ D2 ++ D3 ++ D4.

diagonals( _N, 0, _Change_x, _Y, _Change_y ) -> [];
diagonals( _N, _X, _Change_x, 0, _Change_y ) -> [];
diagonals( N, X, _Change_x, _Y, _Change_y ) when X > N -> [];
diagonals( N, _X, _Change_x, Y, _Change_y ) when Y > N -> [];
diagonals( N, X, Change_x, Y, Change_y ) -> [{X, Y} | diagonals( N, Change_x(X), Change_x, Change_y(Y), Change_y )].

diagonals_add1( N ) -> N + 1.

diagonals_subtract1( N ) -> N - 1.

display_row( Row, N, Board ) ->
[io:fwrite("~s", [display_queen(X, Row, Board)]) || X <- lists:seq(1, N)],
io:nl().

display_queen( X, Y, Board ) -> display_queen( lists:member({X, Y}, Board) ).
display_queen( true ) -> " Q";
display_queen( false ) -> " .".

is_all_queens( N, Board ) -> N =:= erlang:length( Board ).

is_diagonal( _N, [] ) -> false;
is_diagonal( N, [Position | T] ) ->
Diagonals = diagonals( N, Position ),
T =/= (T -- Diagonals)
orelse is_diagonal( N, T ).

is_not_allowed_queen_placement( N, Board ) ->
Pieces = erlang:length( Board ),
{Xs, Ys} = lists:unzip( Board ),
Pieces =/= erlang:length( lists:usort(Xs) )
orelse Pieces =/= erlang:length( lists:usort(Ys) )
orelse is_diagonal( N, Board ).

io:fwrite( "N = ~p. One solution.~n", [N] ),
Board = solve( N ),
display( Board ).
```
Output:
```22> n_queens:task().
N = 4. One solution.
. . Q .
Q . . .
. . . Q
. Q . .
N = 8. One solution.
Q . . . . . . .
. . . . . . Q .
. . . . Q . . .
. . . . . . . Q
. Q . . . . . .
. . . Q . . . .
. . . . . Q . .
. . Q . . . . .
```

### Alternative Version

```%%%For 8X8 chessboard with N queens.
-module(queens).
-export([queens/1]).

queens(0) -> [[]];
queens(N) ->
[[Row | Columns] || Columns <- queens(N-1),
Row <- [1,2,3,4,5,6,7,8] -- Columns,
safe(Row, Columns, 1)].

safe(_Row, [], _N) -> true;
safe(Row, [Column|Columns], N) ->
(Row /= Column + N) andalso (Row /= Column - N) andalso
safe(Row, Columns, (N+1)).
```

## ERRE

```!------------------------------------------------
! QUEENS.R : solve queens problem on a NxN board
!------------------------------------------------

PROGRAM QUEENS

DIM COL%

BEGIN
MAXSIZE%=15
PRINT(TAB(25);" --- PROBLEMA DELLE REGINE --- ")
PRINT
PRINT("Board dimension ";)
INPUT(N%)
PRINT
IF (N%<1 OR N%>MAXSIZE%)
THEN
PRINT("Illegal dimension!!")
ELSE
FOR CURCOLNBR%=1 TO N%
COL%[CURCOLNBR%]=0
END FOR
CURCOLNBR%=1
WHILE CURCOLNBR%>0 DO
PLACEDAQUEEN%=FALSE
I%=COL%[CURCOLNBR%]+1
WHILE (I%<=N%) AND NOT PLACEDAQUEEN% DO
PLACEDAQUEEN%=TRUE
J%=1
WHILE PLACEDAQUEEN% AND (J%<CURCOLNBR%) DO
PLACEDAQUEEN%=COL%[J%]<>I%
J%=J%+1
END WHILE
IF PLACEDAQUEEN%
THEN
DIAGNBR%=I%+CURCOLNBR%
J%=1
WHILE PLACEDAQUEEN% AND (J%<CURCOLNBR%) DO
PLACEDAQUEEN%=(COL%[J%]+J%)<>DIAGNBR%
J%=J%+1
END WHILE
ELSE
END IF
IF PLACEDAQUEEN%
THEN
DIAGNBR%=I%-CURCOLNBR%
J%=1
WHILE PLACEDAQUEEN% AND (J%<CURCOLNBR%) DO
PLACEDAQUEEN%=(COL%[J%]-J%)<>DIAGNBR%
J%=J%+1
END WHILE
ELSE
END IF
IF NOT PLACEDAQUEEN%
THEN
I%=I%+1
ELSE
COL%[CURCOLNBR%]=I%
END IF
END WHILE
IF NOT PLACEDAQUEEN%
THEN
COL%[CURCOLNBR%]=0
CURCOLNBR%=CURCOLNBR%-1
ELSE
IF CURCOLNBR%=N%
THEN
NSOL%=NSOL%+1
PRINT("Soluzione";NSOL%;":";)
FOR I%=1 TO N%
PRINT(COL%[I%];)
END FOR
PRINT
ELSE
CURCOLNBR%=CURCOLNBR%+1
END IF
END IF
END WHILE
PRINT("Search completed")
REPEAT
GET(CH\$)
UNTIL CH\$<>""
END IF
END PROGRAM```

Note: The program prints solutions one per line. This version works well for the PC and the C-64. For PC only you can omit the % integer-type specificator with a `!\$INTEGER` pragma directive.

## F#

```let rec iterate f value = seq {
yield value
yield! iterate f (f value) }

let up i = i + 1
let right i = i
let down i = i - 1

let noCollisionGivenDir solution number dir =
Seq.forall2 (<>) solution (Seq.skip 1 (iterate dir number))

List.forall (noCollisionGivenDir solution number) [ up; right; down ]

let rec extendSolution n ps =
[0..n - 1]
|> List.map (fun num -> num :: ps)

let allSolutions n =
iterate (List.collect (extendSolution n)) [[]]

// Print one solution for the 8x8 case
let printOneSolution () =
allSolutions 8
|> Seq.item 8
|> List.iter (fun rowIndex ->
printf "|"
[0..8] |> List.iter (fun i -> printf (if i = rowIndex then "X|" else " |"))
printfn "")

// Print number of solution for the other cases
let printNumberOfSolutions () =
printfn "Size\tNr of solutions"
[1..11]
|> List.map ((fun i -> Seq.item i (allSolutions i)) >> List.length)
|> List.iteri (fun i cnt -> printfn "%d\t%d" (i+1) cnt)

printOneSolution()

printNumberOfSolutions()
```

The output:

```| | | |X| | | | | |
| |X| | | | | | | |
| | | | | | |X| | |
| | |X| | | | | | |
| | | | | |X| | | |
| | | | | | | |X| |
| | | | |X| | | | |
|X| | | | | | | | |

Size    Nr of solutions
1       1
2       0
3       0
4       2
5       10
6       4
7       40
8       92
9       352
10      724
11      2680
```

## Factor

Works with: Factor version 0.98
```USING: kernel sequences math math.combinatorics formatting io locals ;
IN: queens

: /=  ( x y -- ? )   = not ; inline

:: safe?  ( board q -- ? )
[let  q board nth :> x
q <iota> [
x swap
[ board nth ] keep
q swap -
[ + /= ]
[ - /= ] 3bi and
] all?
] ;

: solution? ( board -- ? )
dup length <iota> [ dupd safe? ] all? nip ;

: queens ( n -- l )
<iota> all-permutations [ solution? ] filter ;

: .queens ( n -- )
queens
[
[ 1 + "%d " printf ] each nl
] each ;
```

## Forth

```variable solutions
variable nodes

: bits ( n -- mask ) 1 swap lshift 1- ;
: lowBit  ( mask -- bit ) dup negate and ;
: lowBit- ( mask -- bits ) dup 1- and ;

: next3 ( dl dr f files -- dl dr f dl' dr' f' )
invert >r
2 pick r@ and 2* 1+
2 pick r@ and 2/
2 pick r> and ;

: try ( dl dr f -- )
dup if
1 nodes +!
dup 2over and and
begin ?dup while
dup >r lowBit next3 recurse r> lowBit-
repeat
else 1 solutions +! then
drop 2drop ;

: queens ( n -- )
0 solutions ! 0 nodes !
-1 -1 rot bits try
solutions @ . ." solutions, " nodes @ . ." nodes" ;

8 queens  \ 92 solutions, 1965 nodes
```

### Alternate solution adapted from FD-V02N1.pdf

```\ http://www.forth.org/fd/FD-V02N1.pdf
VOCABULARY nqueens ALSO nqueens DEFINITIONS

8 constant queens

\ Nqueen solution from FD-V02N1.pdf
: 1array CREATE 0 DO 1 , LOOP DOES> SWAP CELLS + ;
queens 1array a \ a,b & c: workspaces for solutions
queens 2* 1array b
queens 2* 1array c
queens 1array x \ trial solutions

: safe ( c i -- n )
SWAP
2DUP - queens 1- + c @ >R
2DUP + b @ >R
DROP a @ R> R> * * ;

: mark ( c i -- )
SWAP
2DUP - queens 1- + c 0 swap !
2DUP + b 0 swap !
DROP a 0 swap ! ;

: unmark ( c i -- )
SWAP
2DUP - queens 1- + c 1 swap !
2DUP + b 1 swap !
DROP a 1 swap ! ;

VARIABLE tries
VARIABLE sols

: .cols queens 0 DO I x @ 1+ 5 .r loop ;
: .sol ." Found on try " tries @ 6 .R .cols cr ;

: try
queens 0
DO 1 tries +!
DUP I safe
IF DUP I mark
DUP I SWAP x !
DUP queens 1- < IF DUP 1+ RECURSE ELSE sols ++ .sol THEN
DUP I unmark
THEN
LOOP DROP ;

: go 0 tries ! CR 0 try CR sols @ . ." solutions Found, for n = " queens . ;
go
```

## Fortran

Works with: Fortran version 95 and later

Using a back tracking method to find one solution

```program Nqueens
implicit none

integer, parameter :: n = 8  ! size of board
integer :: file = 1, rank = 1, queens = 0
integer :: i
logical :: board(n,n) = .false.

do while (queens < n)
board(file, rank) = .true.
if(is_safe(board, file, rank)) then
queens = queens + 1
file = 1
rank = rank + 1
else
board(file, rank) = .false.
file = file + 1
do while(file > n)
rank = rank - 1
if (rank < 1) then
write(*, "(a,i0)") "No solution for n = ", n
stop
end if
do i = 1, n
if (board(i, rank)) then
file = i
board(file, rank) = .false.
queens = queens - 1
file = i + 1
exit
end if
end do
end do
end if
end do

call Printboard(board)

contains

function is_safe(board, file, rank)
logical :: is_safe
logical, intent(in) :: board(:,:)
integer, intent(in) :: file, rank
integer :: i, f, r

is_safe = .true.
do i = rank-1, 1, -1
if(board(file, i)) then
is_safe = .false.
return
end if
end do

f = file - 1
r = rank - 1
do while(f > 0 .and. r > 0)
if(board(f, r)) then
is_safe = .false.
return
end if
f = f - 1
r = r - 1
end do

f = file + 1
r = rank - 1
do while(f <= n .and. r > 0)
if(board(f, r)) then
is_safe = .false.
return
end if
f = f + 1
r = r - 1
end do
end function

subroutine Printboard(board)
logical, intent(in) :: board(:,:)
character(n*4+1) :: line
integer :: f, r

write(*, "(a, i0)") "n = ", n
line = repeat("+---", n) // "+"
do r = 1, n
write(*, "(a)") line
do f = 1, n
if(board(f, r)) then
write(*, "(a)", advance="no") " Q "
else if(mod(f+r, 2) == 0) then
else
end if
end do
write(*, "(a)") "|"
end do
write(*, "(a)") line
end subroutine
end program
```
Output:
for 8, 16 and 32 queens
```n = 8
+---+---+---+---+---+---+---+---+
| Q |###|   |###|   |###|   |###|
+---+---+---+---+---+---+---+---+
|###|   |###|   | Q |   |###|   |
+---+---+---+---+---+---+---+---+
|   |###|   |###|   |###|   | Q |
+---+---+---+---+---+---+---+---+
|###|   |###|   |###| Q |###|   |
+---+---+---+---+---+---+---+---+
|   |###| Q |###|   |###|   |###|
+---+---+---+---+---+---+---+---+
|###|   |###|   |###|   | Q |   |
+---+---+---+---+---+---+---+---+
|   | Q |   |###|   |###|   |###|
+---+---+---+---+---+---+---+---+
|###|   |###| Q |###|   |###|   |
+---+---+---+---+---+---+---+---+

n = 16
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| Q |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|###|   | Q |   |###|   |###|   |###|   |###|   |###|   |###|   |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|   |###|   |###| Q |###|   |###|   |###|   |###|   |###|   |###|
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|###| Q |###|   |###|   |###|   |###|   |###|   |###|   |###|   |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|   |###|   |###|   |###|   |###|   |###|   |###| Q |###|   |###|
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|###|   |###|   |###|   |###|   | Q |   |###|   |###|   |###|   |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|   |###|   |###|   |###|   |###|   |###|   |###|   | Q |   |###|
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|###|   |###|   |###|   |###|   |###|   |###| Q |###|   |###|   |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|   |###|   |###|   |###|   |###|   |###|   |###|   |###| Q |###|
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|###|   |###|   |###| Q |###|   |###|   |###|   |###|   |###|   |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   | Q |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|###|   |###|   |###|   | Q |   |###|   |###|   |###|   |###|   |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|   |###|   | Q |   |###|   |###|   |###|   |###|   |###|   |###|
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|###|   |###|   |###|   |###|   |###|   | Q |   |###|   |###|   |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|   |###|   |###|   |###|   | Q |   |###|   |###|   |###|   |###|
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|###|   |###|   |###|   |###|   |###| Q |###|   |###|   |###|   |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

n = 32
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| Q |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|###|   | Q |   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|   |###|   |###| Q |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|###| Q |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|   |###|   | Q |   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|###|   |###|   |###|   |###|   | Q |   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|   |###|   |###|   |###|   |###|   |###| Q |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|###|   |###|   |###|   |###|   |###|   |###|   | Q |   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|   |###|   |###|   |###|   |###|   |###|   |###|   |###| Q |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|###|   |###|   |###| Q |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   | Q |   |###|   |###|   |###|   |###|   |###|   |###|   |###|
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###| Q |###|   |###|   |###|   |###|   |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   | Q |   |###|   |###|   |###|
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###| Q |###|   |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###| Q |###|   |###|   |###|   |###|
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   | Q |   |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   | Q |   |###|   |###|
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###| Q |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###| Q |###|   |###|   |###|
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   | Q |   |###|   |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   | Q |   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   | Q |   |###|   |###|   |###|   |###|   |###|   |###|   |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|   |###|   |###|   |###|   |###|   | Q |   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|###|   |###|   |###|   |###| Q |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###| Q |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|###|   |###|   |###|   |###|   |###|   |###| Q |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###| Q |###|   |###|   |###|   |###|   |###|   |###|
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|###|   |###|   |###|   | Q |   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|   |###|   |###|   |###|   |###|   |###|   |###|   | Q |   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   | Q |   |###|   |###|   |###|   |###|   |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   | Q |   |###|   |###|   |###|   |###|   |###|   |###|
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###|   |###| Q |###|   |###|   |###|   |###|   |###|   |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+```

### Alternate Fortran 77 solution

```C This one implements depth-first backtracking.
C See the 2nd program for Scheme on the "Permutations" page for the
C main idea.
C As is, the program only prints the number of n-queens configurations.
C To print also the configurations, uncomment the line after label 80.
program queens
implicit integer(a-z)
parameter(l=18)
dimension a(l),s(l),u(4*l-2)
do 10 i=1,l
10 a(i)=i
do 20 i=1,4*l-2
20 u(i)=0
do 110 n=1,l
m=0
i=1
r=2*n-1
go to 40
30 s(i)=j
u(p)=1
u(q+r)=1
i=i+1
40 if(i.gt.n) go to 80
j=i
50 z=a(i)
y=a(j)
p=i-y+n
q=i+y-1
a(i)=y
a(j)=z
if((u(p).eq.0).and.(u(q+r).eq.0)) goto 30
60 j=j+1
if(j.le.n) go to 50
70 j=j-1
if(j.eq.i) go to 90
z=a(i)
a(i)=a(j)
a(j)=z
go to 70
80 m=m+1
C     print *,(a(k),k=1,n)
90 i=i-1
if(i.eq.0) go to 100
p=i-a(i)+n
q=i+a(i)-1
j=s(i)
u(p)=0
u(q+r)=0
go to 60
100 print *,n,m
110 continue
end

C Output
C          1           1
C          2           0
C          3           0
C          4           2
C          5          10
C          6           4
C          7          40
C          8          92
C          9         352
C         10         724
C         11        2680
C         12       14200
C         13       73712
C         14      365596
C         15     2279184
C         16    14772512
C         17    95815104
C         18   666090624
```
```!The preceding program implements recursion using arrays, since Fortran 77 does not allow recursive
!functions. The same algorithm is much easier to follow in Fortran 90, using the RECURSIVE keyword.
!Like previously, the program only counts solutions. It's pretty straightforward to adapt it to print
!them too: one has to replace the 'm = m + 1' instruction with a PRINT statement.

function numq(n)
implicit none
integer :: i, n, m, a(n), numq
logical :: up(2*n - 1), down(2*n - 1)
do i = 1, n
a(i) = i
end do
up = .true.
down = .true.
m = 0
call sub(1)
numq = m
contains
recursive subroutine sub(i)
integer :: i, j, k, p, q, s
do k = i, n
j = a(k)
p = i + j - 1
q = i - j + n
if(up(p) .and. down(q)) then
if(i == n) then
m = m + 1
else
up(p) = .false.
down(q) = .false.
s = a(i)
a(i) = a(k)
a(k) = s
call sub(i + 1)
up(p) = .true.
down(q) = .true.
s = a(i)
a(i) = a(k)
a(k) = s
end if
end if
end do
end subroutine
end function

program queens
implicit none
integer :: numq, n, m
do n = 4, 16
m = numq(n)
print *, n, m
end do
end program
```

### Alternate Fortran 95 solution with OpenMP

This code is useful mainly for counting solutions. Here we use the same algorithm as with Fortran 77, with an optimization: because of symmetry of the chess board, computations are divided by two. The remaining is parallelized with OpenMP. The loop is done on the valid combinations of queens in the first two columns. The original algorithm is slightly changed to start backtracking from column three.

If using GCC, compile with gfortran -O2 -fopenmp queens.f90. With Absoft Pro Fortran, af90 -O2 -openmp queens.f90, and with Intel Fortran, ifort /fast /Qopenmp queens.f90.

With some versions of GCC the function OMP_GET_WTIME is not known, which seems to be a bug. Then it's enough to comment out the two calls, and the program won't display timings.

```program queens
use omp_lib
implicit none
integer, parameter :: long = selected_int_kind(17)
integer, parameter :: l = 18
integer, parameter :: nthreads = 16    ! Change to suit your processor
integer :: n, i, j, a(l*l, 2), k, p, q
integer(long) :: s, b(l*l)
real(kind(1d0)) :: t1, t2
! Edit : Added OPEN MP calls to set number of threads
CALL OMP_SET_DYNAMIC(.TRUE.)
do n = 6, l
k = 0
p = n/2
q = mod(n, 2)*(p + 1)
do i = 1, n
do j = 1, n
if ((abs(i - j) > 1) .and. ((i <= p) .or. ((i == q) .and. (j < i)))) then
k = k + 1
a(k, 1) = i
a(k, 2) = j
end if
end do
end do
s = 0
t1 = omp_get_wtime()
!\$omp parallel do schedule(dynamic)
do i = 1, k
b(i) = pqueens(n, a(i, 1), a(i, 2))
end do
!\$omp end parallel do
t2 = omp_get_wtime()
print "(I4, I12, F12.3)", n, 2*sum(b(1:k)), t2 - t1
end do

contains
function pqueens(n, k1, k2) result(m)
implicit none
integer(long) :: m
integer, intent(in) :: n, k1, k2
integer, parameter :: l = 20
integer :: a(l), s(l), u(4*l - 2)
integer :: i, j, y, z, p, q, r

do i = 1, n
a(i) = i
end do

do i = 1, 4*n - 2
u(i) = 0
end do

m = 0
r = 2*n - 1
if (k1 == k2) return

p = 1 - k1 + n
q = 1 + k1 - 1
if ((u(p) /= 0) .or. (u(q + r) /= 0)) return

u(p) = 1
u(q + r) = 1
z = a(1)
a(1) = a(k1)
a(k1) = z
p = 2 - k2 + n
q = 2 + k2 - 1
if ((u(p) /= 0) .or. (u(q + r) /= 0)) return

u(p) = 1
u(q + r) = 1
if (k2 /= 1) then
z = a(2)
a(2) = a(k2)
a(k2) = z
else
z = a(2)
a(2) = a(k1)
a(k1) = z
end if
i = 3
go to 40

30 s(i) = j
u(p) = 1
u(q + r) = 1
i = i + 1
40 if (i > n) go to 80

j = i

50 z = a(i)
y = a(j)
p = i - y + n
q = i + y - 1
a(i) = y
a(j) = z
if ((u(p) == 0) .and. (u(q + r) == 0)) go to 30

60 j = j + 1
if (j <= n) go to 50

70 j = j - 1
if (j == i) go to 90

z = a(i)
a(i) = a(j)
a(j) = z
go to 70

!valid queens position found
80 m = m + 1

90 i = i - 1
if (i == 2) return

p = i - a(i) + n
q = i + a(i) - 1
j = s(i)
u(p) = 0
u(q + r) = 0
go to 60
end function
end program
```

### Fortran 2008 in a Lisp-like fashion

Works with: Fortran version 2008 and later

The following program solves, stores, and prints all solutions to the n-queens problem, for board sizes given on the command line. To compile it, you need my modules that employ Fortran 2008’s type polymorphism to support Lisp-like CONS-pairs. The modules (and this program) are available at https://sourceforge.net/p/chemoelectric/fortran-modules along with a GNU makefile, all under a permissive free software license. The makefile is written for GNU Fortran; compiler version 11.2.1 works. The programming style is essentially functional programming, and solutions are stored as a linked list of linked lists. One might notice how circular lists are used within the code to overcome Fortran’s limited ability to do closures.

Part of the intent here is to show that Fortran can do quite a few things people would not think it could, if it is given adequate library support.

```program example__n_queens

use, intrinsic :: iso_fortran_env, only: output_unit

use, non_intrinsic :: garbage_collector
use, non_intrinsic :: cons_pairs

implicit none

! .true. is good for testing that necessary values are rooted.
! .false. to collect garbage only when the heap reaches a limit.
logical :: aggressive_garbage_collection = .true.

integer :: arg_count
integer :: stat
character(80) :: arg

type(gcroot_t) :: board_sizes

arg_count = command_argument_count ()
if (arg_count < 1) then
call print_usage (output_unit)
else
board_sizes = nil
block
integer :: i
integer :: board_size
do i = 1, arg_count
call get_command_argument (i, arg)
read (arg, *, iostat = stat) board_size
if (stat /= 0 .or. board_size < 1) then
board_size = -1
end if
board_sizes = cons (board_size, board_sizes)
end do
board_sizes = reversex (board_sizes)
end block

if (is_member (int_eq, -1, board_sizes)) then
call print_usage (output_unit)
else
! Use pair_for_each as a way to distinguish the last
! BOARD_SIZE from the others. The last entry will be the final
! pair, and so its CDR will *not* be a pair.
call pair_for_each (find_and_print_all_solutions, &
&              circular_list (output_unit), &
&              board_sizes)
end if
end if

contains

subroutine print_usage (outp)
integer, intent(in) :: outp

write (outp, '("Usage: example__n_queens BOARD_SIZE [BOARD_SIZE...]")')
write (outp, '("Each BOARD_SIZE must be at least 1.")')
write (outp, '("For each BOARD_SIZE, all solutions are computed before any is printed.")')
end subroutine print_usage

subroutine find_and_print_all_solutions (outp_pair, board_sizes)
class(*), intent(in) :: outp_pair
class(*), intent(in) :: board_sizes

integer :: n_outp
type(gcroot_t) :: all_solutions

n_outp = int_cast (car (outp_pair))

all_solutions = find_all_solutions (car (board_sizes))
call check_garbage
call print_all_solutions (n_outp, car (board_sizes), all_solutions)
call check_garbage
if (is_pair (cdr (board_sizes))) then
! Space between one BOARD_SIZE and another.
write (n_outp, '()')
end if
end subroutine find_and_print_all_solutions

function find_all_solutions (board_size) result (all_solutions)
class(*), intent(in) :: board_size
type(cons_t) :: all_solutions

class(*), allocatable :: solutions

call find_solutions_from_ranks_so_far (board_size, nil, solutions)
all_solutions = solutions
end function find_all_solutions

recursive subroutine find_solutions_from_ranks_so_far (board_size, ranks_so_far, solutions)
class(*), intent(in) :: board_size
class(*), intent(in) :: ranks_so_far
class(*), allocatable, intent(out) :: solutions

type(cons_t) :: ranks

if (length (ranks_so_far) == int_cast (board_size)) then
solutions = list (ranks_so_far)
else
ranks = find_legal_ranks_for_file (int_cast (board_size), ranks_so_far)
solutions = concatenatex (map (find_solutions_from_ranks_so_far,                  &
&                         circular_list (board_size),                        &
&                         map (kons, ranks, circular_list (ranks_so_far))))
end if
end subroutine find_solutions_from_ranks_so_far

function find_legal_ranks_for_file (board_size, ranks_so_far) result (ranks)
!
! Return a list of all the ranks in the next file, under the
! constraint that a queen placed in the position not be under
! attack.
!
integer, intent(in) :: board_size
class(*), intent(in) :: ranks_so_far
type(cons_t) :: ranks

ranks = iota (board_size, 1) ! All the possible ranks.
ranks = remove_illegal_ranks (ranks, ranks_so_far)
end function find_legal_ranks_for_file

function remove_illegal_ranks (new_ranks, ranks_so_far) result (legal_ranks)
class(*), intent(in) :: new_ranks
class(*), intent(in) :: ranks_so_far
type(cons_t) :: legal_ranks

legal_ranks = filter_map (keep_legal_rank, new_ranks, &
&                    circular_list (ranks_so_far))
end function remove_illegal_ranks

subroutine keep_legal_rank (rank, ranks_so_far, retval)
class(*), intent(in) :: rank
class(*), intent(in) :: ranks_so_far
class(*), allocatable, intent(out) :: retval

if (rank_is_legal (rank, ranks_so_far)) then
retval = rank
else
retval = .false.
end if
end subroutine keep_legal_rank

function rank_is_legal (new_rank, ranks_so_far) result (bool)
class(*), intent(in) :: new_rank
class(*), intent(in) :: ranks_so_far
logical :: bool

integer :: new_file
type(cons_t) :: files_so_far

new_file = int (length (ranks_so_far)) + 1
files_so_far = iota (new_file - 1, new_file - 1, -1)
bool = every (these_two_queens_are_nonattacking, &
&        circular_list (new_file),          &
&        circular_list (new_rank),          &
&        files_so_far,                      &
&        ranks_so_far)
end function rank_is_legal

function these_two_queens_are_nonattacking (file1, rank1, file2, rank2) result (bool)
class(*), intent(in) :: file1, rank1
class(*), intent(in) :: file2, rank2
logical :: bool

integer :: f1, r1
integer :: f2, r2

! The rank and the two diagonals must not be the same. (The files
! are known to be different.)

f1 = int_cast (file1)
r1 = int_cast (rank1)
f2 = int_cast (file2)
r2 = int_cast (rank2)

bool = (r1 /= r2 .and. r1 + f1 /= r2 + f2 .and. r1 - f1 /= r2 - f2)
end function these_two_queens_are_nonattacking

subroutine print_all_solutions (outp, board_size, all_solutions)
class(*), intent(in) :: outp
class(*), intent(in) :: board_size
class(*), intent(in) :: all_solutions

integer(size_kind) :: n

n = length (all_solutions)
write (int_cast (outp), '("For a board ", I0, " by ", I0, ", ")', advance = 'no') &
&    int_cast (board_size), int_cast (board_size)
if (n == 1) then
write (int_cast (outp), '("there is ", I0, " solution.")') n
else
write (int_cast (outp), '("there are ", I0, " solutions.")') n
end if
call for_each (print_spaced_solution, circular_list (outp), &
&         circular_list (board_size), all_solutions)
end subroutine print_all_solutions

subroutine print_spaced_solution (outp, board_size, solution)
class(*), intent(in) :: outp
class(*), intent(in) :: board_size
class(*), intent(in) :: solution

write (int_cast (outp), '()', advance = 'yes')
call print_solution (outp, board_size, solution)
end subroutine print_spaced_solution

subroutine print_solution (outp, board_size, solution)
class(*), intent(in) :: outp
class(*), intent(in) :: board_size
class(*), intent(in) :: solution

integer :: n_outp
integer :: n_board_size
integer :: rank
integer :: file
integer :: file_of_queen

n_outp = int_cast (outp)
n_board_size = int_cast (board_size)

do rank = n_board_size, 1, -1
do file = 1, n_board_size
write (n_outp, '("----")', advance = 'no')
end do
write (n_outp, '("-")', advance = 'yes')

file_of_queen = n_board_size - int (list_index0 (int_eq, circular_list (rank), solution))

do file = 1, n_board_size
if (file == file_of_queen) then
write (n_outp, '("| Q ")', advance = 'no')
else
write (n_outp, '("|   ")', advance = 'no')
end if
end do
write (n_outp, '("|")', advance = 'yes')
end do

do file = 1, n_board_size
write (n_outp, '("----")', advance = 'no')
end do
write (n_outp, '("-")', advance = 'yes')
end subroutine print_solution

subroutine kons (x, y, xy)
class(*), intent(in) :: x
class(*), intent(in) :: y
class(*), allocatable, intent(out) :: xy

xy = cons (x, y)
end subroutine kons

pure function int_cast (x) result (val)
class(*), intent(in) :: x
integer :: val

select type (x)
type is (integer)
val = x
class default
error stop
end select
end function int_cast

pure function int_eq (x, y) result (bool)
class(*), intent(in) :: x
class(*), intent(in) :: y
logical :: bool

bool = (int_cast (x) == int_cast (y))
end function int_eq

subroutine check_garbage
if (aggressive_garbage_collection) then
call collect_garbage_now
else
call check_heap_size
end if
end subroutine check_garbage

end program example__n_queens
```
Output:
\$ ./example__n_queens 1 2 3 4
```For a board 1 by 1, there is 1 solution.

-----
| Q |
-----

For a board 2 by 2, there are 0 solutions.

For a board 3 by 3, there are 0 solutions.

For a board 4 by 4, there are 2 solutions.

-----------------
|   | Q |   |   |
-----------------
|   |   |   | Q |
-----------------
| Q |   |   |   |
-----------------
|   |   | Q |   |
-----------------

-----------------
|   |   | Q |   |
-----------------
| Q |   |   |   |
-----------------
|   |   |   | Q |
-----------------
|   | Q |   |   |
-----------------
```

## FreeBASIC

Get slower for N > 14

```' version 13-04-2017
' compile with: fbc -s console
Dim Shared As ULong count, c()

Sub n_queens(row As ULong, n As ULong, show As ULong = 0)

Dim As ULong x, y

For x = 1 To n

For y = 1 To row -1
If c(y) = x OrElse ((row - y) - Abs(x - c(y))) = 0 Then
Continue For, For
End If
Next

c(row) = x
If row < n Then
n_queens(row +1 , n, show)
Else
count += 1

If show <> 0 Then
For y = 1 To n
Print Using "###"; c(y);
Next
Print
End If

End If

Next

End Sub

' ------=< MAIN >=------

Dim As ULong n = 5
ReDim c(n)
' n_queens(1, n, show = 0 only show total | show <> 0 show every solution
n_queens(1, n, 1)
Print Using "## x ## board, ##### solutions"; n; n; count
Print

For n = 1 To 14
ReDim c(n)
count = 0
n_queens(1, n)
Print Using "A ## x ## board has ######## solutions"; n; n; count
Next

' empty keyboard buffer
While Inkey <> "" : Wend
Print : Print "hit any key to end program"
Sleep
End```
Output:
```  1  3  5  2  4
1  4  2  5  3
2  4  1  3  5
2  5  3  1  4
3  1  4  2  5
3  5  2  4  1
4  1  3  5  2
4  2  5  3  1
5  2  4  1  3
5  3  1  4  2
5 x  5 board,    10 solutions

A  1 x  1 board has        1 solutions
A  2 x  2 board has        0 solutions
A  3 x  3 board has        0 solutions
A  4 x  4 board has        2 solutions
A  5 x  5 board has       10 solutions
A  6 x  6 board has        4 solutions
A  7 x  7 board has       40 solutions
A  8 x  8 board has       92 solutions
A  9 x  9 board has      352 solutions
A 10 x 10 board has      724 solutions
A 11 x 11 board has     2680 solutions
A 12 x 12 board has    14200 solutions
A 13 x 13 board has    73712 solutions
A 14 x 14 board has   365596 solutions```

### Alternate version : recursive

```Sub aux(n As Integer, i As Integer, a() As Integer, _
u() As Integer, v() As Integer, ByRef m As LongInt)

Dim As Integer j, k, p, q
If i > n Then
m += 1
For k = 1 To n : Print a(k); : Next : Print
Else
For j = i To n
k = a(j)
p = i - k + n
q = i + k - 1
If u(p) And v(q) Then
u(p) = 0 : v(q) = 0
a(j) = a(i) : a(i) = k
aux(n, i + 1, a(), u(), v(), m)
u(p) = 1 : v(q) = 1
a(i) = a(j) : a(j) = k
End If
Next
End If
End Sub

Dim As Integer n, i
Dim m As LongInt = 1
If Command(1) <> "" Then
n = CInt(Command(1))
ReDim a(1 To n) As Integer
ReDim u(1 To 2 * n - 1) As Integer
ReDim v(1 To 2 * n - 1) As Integer
For i = 1 To n
a(i) = i
Next
For i = 1 To 2 * n - 1
u(i) = 1
v(i) = 1
Next
m = 0
aux(n, 1, a(), u(), v(), m)
Print m
End If```

### Alternate version : iterative

```Dim As Integer n, i, j, k, p, q
Dim m As LongInt = 0

If Command(1) <> "" Then
n = CInt(Command(1))
ReDim a(1 To n) As Integer
ReDim s(1 To n) As Integer
ReDim u(1 To 2 * n - 1) As Integer
ReDim v(1 To 2 * n - 1) As Integer
For i = 1 To n
a(i) = i
Next
For i = 1 To 2 * n - 1
u(i) = 1
v(i) = 1
Next
m = 0
i = 1
L1: If i > n Then
m += 1
For k = 1 To n : Print a(k); : Next : Print
Goto L4
End If
j = i
L2: k = a(j)
p = i - k + n
q = i + k - 1
If u(p) And v(q) Then
u(p) = 0 : v(q) = 0
a(j) = a(i) : a(i) = k
s(i) = j
i += 1
Goto L1
End If
L3: j += 1 : If j <= n Goto L2
L4: i -= 1 : If i = 0 Then Print m : End
j = s(i)
k = a(i) : a(i) = a(j) : a(j) = k
p = i - k + n
q = i + k - 1
u(p) = 1 : v(q) = 1
Goto L3
End If```

## Frink

This example uses Frink's built-in `array.permute[]` method to generate possible permutations of the board efficiently.

```solution[board] :=
{
for q = 0 to length[board] - 1
for c = q+1 to length[board] - 1
if board@q == board@c + (c - q) or board@q == board@c - (c - q)
return false
return true
}

for b = array[1 to 8].permute[]
if solution[b]
println[b]```

## Fōrmulæ

Fōrmulæ programs are not textual, visualization/edition of programs is done showing/manipulating structures but not text. Moreover, there can be multiple visual representations of the same program. Even though it is possible to have textual representation —i.e. XML, JSON— they are intended for storage and transfer purposes more than visualization and edition.

Programs in Fōrmulæ are created/edited online in its website.

In this page you can see and run the program(s) related to this task and their results. You can also change either the programs or the parameters they are called with, for experimentation, but remember that these programs were created with the main purpose of showing a clear solution of the task, and they generally lack any kind of validation.

Solution

The following function:

• Is able to calculate solution for chessboards of any size (but it is slow for big chessboards)
• It does not detect rotated or reflected solutions

This is an example of backtracking:

Improvement. The following functions calls the previous one, but shows the solution on a more friendly way

## GAP

Translation of Fortran 77. See also alternate Python implementation. One function to return the number of solutions, another to return the list of permutations.

```NrQueens := function(n)
local a, up, down, m, sub;
a := [1 .. n];
up := ListWithIdenticalEntries(2*n - 1, true);
down := ListWithIdenticalEntries(2*n - 1, true);
m := 0;
sub := function(i)
local j, k, p, q;
for k in [i .. n] do
j := a[k];
p := i + j - 1;
q := i - j + n;
if up[p] and down[q] then
if i = n then
m := m + 1;
else
up[p] := false;
down[q] := false;
a[k] := a[i];
a[i] := j;
sub(i + 1);
up[p] := true;
down[q] := true;
a[i] := a[k];
a[k] := j;
fi;
fi;
od;
end;
sub(1);
return m;
end;

Queens := function(n)
local a, up, down, v, sub;
a := [1 .. n];
up := ListWithIdenticalEntries(2*n - 1, true);
down := ListWithIdenticalEntries(2*n - 1, true);
v := [];
sub := function(i)
local j, k, p, q;
for k in [i .. n] do
j := a[k];
p := i + j - 1;
q := i - j + n;
if up[p] and down[q] then
if i = n then
else
up[p] := false;
down[q] := false;
a[k] := a[i];
a[i] := j;
sub(i + 1);
up[p] := true;
down[q] := true;
a[i] := a[k];
a[k] := j;
fi;
fi;
od;
end;
sub(1);
return v;
end;

NrQueens(8);
a := Queens(8);;
PrintArray(PermutationMat(PermList(a), 8));

[ [  1,  0,  0,  0,  0,  0,  0,  0 ],
[  0,  0,  0,  0,  1,  0,  0,  0 ],
[  0,  0,  0,  0,  0,  0,  0,  1 ],
[  0,  0,  0,  0,  0,  1,  0,  0 ],
[  0,  0,  1,  0,  0,  0,  0,  0 ],
[  0,  0,  0,  0,  0,  0,  1,  0 ],
[  0,  1,  0,  0,  0,  0,  0,  0 ],
[  0,  0,  0,  1,  0,  0,  0,  0 ] ]
```

## Go

### Niklaus Wirth algorithm (Wikipedia)

```// A fairly literal translation of the example program on the referenced
// WP page.  Well, it happened to be the example program the day I completed
// the task.  It seems from the WP history that there has been some churn
// in the posted example program.  The example program of the day was in
// Pascal and was credited to Niklaus Wirth, from his "Algorithms +
// Data Structures = Programs."
package main

import "fmt"

var (
i int
q bool
a bool
b bool
c bool // offset by 7 relative to the Pascal version
x int
)

func try(i int) {
for j := 1; ; j++ {
q = false
if a[j] && b[i+j] && c[i-j+7] {
x[i] = j
a[j] = false
b[i+j] = false
c[i-j+7] = false
if i < 8 {
try(i + 1)
if !q {
a[j] = true
b[i+j] = true
c[i-j+7] = true
}
} else {
q = true
}
}
if q || j == 8 {
break
}
}
}

func main() {
for i := 1; i <= 8; i++ {
a[i] = true
}
for i := 2; i <= 16; i++ {
b[i] = true
}
for i := 0; i <= 14; i++ {
c[i] = true
}
try(1)
if q {
for i := 1; i <= 8; i++ {
fmt.Println(i, x[i])
}
}
}
```
Output:
```1 1
2 5
3 8
4 6
5 3
6 7
7 2
8 4
```

### Refactored Niklaus Wirth algorithm (clearer/Go friendly solution)

```/*
* N-Queens Problem
*
* For an NxN chess board, 'safely' place a chess queen in every column and row such that none can attack another.
* This solution is based Wirth Pascal solution, although a tad cleaner, thus easier to understand as it uses Go/C
* style indexing and naming, and also prints the Queen using a Unicode 'rune' (which other languages do not handle natively).
*
* N rows by N columns are number left to right top to bottom 0 - 7
*
* There are 2N-1 diagonals (showing an 8x8)
*  the upper-right to lower-left are numbered row + col that is:
*    0   1   2   3   4   5   6   7
*    1   2   3   4   5   6   7   8
*    2   3   4   5   6   7   8   9
*    3   4   5   6   7   8   9  10
*    4   5   6   7   8   9  10  11
*    5   6   7   8   9  10  11  12
*    6   7   8   9  10  11  12  13
*    7   8   9  10  11  12  13  14
*
*	the upper-left to lower-right are numbered N-1 + row - col
*    7   6   5   4   3   2   1   0
*    8   7   6   5   4   3   2   1
*    9   8   7   6   5   4   3   2
*   10   9   8   7   6   5   4   3
*   11  10   9   8   7   6   5   4
*   12  11  10   9   8   7   6   5
*   13  12  11  10   9   8   7   6
*   14  13  12  11  10   9   8   7
*/

package main

import "fmt"

const	N	= 8
const	HAS_QUEEN  = false
const	EMPTY  = true
const	UNASSIGNED = -1
const   white_queen = '\u2655'

var	row_num[N]int	// results, indexed by row will be the column where the queen lives (UNASSIGNED) is empty
var	right_2_left_diag[(2*N-1)]bool	// T if no queen in diag[idx]: row i, column col is diag i+col
var	left_2_right_diag[(2*N-1)]bool //  T is no queen in diag[idx], row i, column col is N-1 + i-col

func printresults() {
for col := 0; col < N; col++ {
if col != 0 {
fmt.Printf(" ");
}
fmt.Printf("%d,%d", col, row_num[col])
}
fmt.Printf("\n");
for  row := 0; row < N; row++ {
for col := 0; col < N; col++ {
if col == row_num[row] {
fmt.Printf(" %c ", white_queen)
} else {
fmt.Printf(" . ")
}
}
fmt.Printf("\n")
}
}

/*
* save a queen on the board by saving where we think it should go, and marking the diagonals as occupied
*/

func savequeen(row int, col int) {
row_num[row] = col	// save queen column for this row
right_2_left_diag[row+col] = HAS_QUEEN 	// mark forward diags as occupied
left_2_right_diag[row-col+(N-1)] = HAS_QUEEN	// mark backward diags as occupied
}

/*
* backout a previously saved queen by clearing where we put it, and marking the diagonals as empty
*/

func clearqueen(row int, col int) {
row_num[row] = UNASSIGNED
right_2_left_diag[row+col] = EMPTY
left_2_right_diag[row-col+(N-1)] = EMPTY
}

/*
* for each column try the solutions
*/
func trycol(col int)  bool {
// check each row to look for the first empty row that does not have a diagonal in use too
for row := 0; row < N; row++ {
if row_num[row] == UNASSIGNED &&	// has the row been used yet?
right_2_left_diag[row+col] == EMPTY &&	// check for the forward diags
left_2_right_diag[row-col+(N-1)] == EMPTY {	// check for the backwards diags
savequeen(row, col)	// this is a possible solution
// Tricky part here:  going forward thru the col up to but not including the rightmost one
// if this fails, we are done, no need to search any more
if col < N-1 && !trycol(col+1) {
// ok this did not work - we need to try a different row, so undo the guess
clearqueen(row, col)
} else {
// we have a solution on this row/col, start popping the stack.
return true
}
}
}
return false // not a solution for this col, pop the stack, undo the last guess, and try the next one
}

func main() {
for i := 0; i < N ; i++ {
row_num[i] = UNASSIGNED
}
for i := 0; i < 2*N-1 ; i++ {
right_2_left_diag[i] = EMPTY
}
for i := 0; i < 2*N-1 ; i++ {
left_2_right_diag[i] = EMPTY
}
trycol(0)
printresults()
}
```
Output:
```0,0 1,6 2,4 3,7 4,1 5,3 6,5 7,2
♕  .  .  .  .  .  .  .
.  .  .  .  .  .  ♕  .
.  .  .  .  ♕  .  .  .
.  .  .  .  .  .  .  ♕
.  ♕  .  .  .  .  .  .
.  .  .  ♕  .  .  .  .
.  .  .  .  .  ♕  .  .
.  .  ♕  .  .  .  .  .
```

### Dancing Links / Algorithm X

Using Knuth's dancing links technique to implement his Knuth's Algorithm X. The Go code for this technique is in the dlx packge.

```package main

import (
"flag"
"fmt"
"log"
"os"
"time"

"rosettacode.org/dlx" // or where ever you put the dlx package
)

func main() {
log.SetPrefix("N-queens: ")
log.SetFlags(0)
profile := flag.Bool("profile", false, "show DLX profile")
flag.Parse()

for N := 2; N <= 18; N++ {
err := nqueens(N, N == 8, *profile)
if err != nil {
log.Fatal(err)
}
}
}

func nqueens(N int, printFirst, profile bool) error {
// Build a new DLX matrix with 2N primary columns and 4N-6 secondary
// columns: R0..R(N-1), F0..F(N-1), A1..A(2N-3), B1..B(2N-3).
// We also know the number of cells and solution rows required.
m := dlx.NewWithHint(2*N, 4*N-6, N*N*4-4, 8)

s := solution{
N:          N,
renumFwd:   make([]int, 0, 2*N),
renumBack:  make([]int, 2*N),
printFirst: printFirst,
}

// column indexes
iR0 := 0
iF0 := iR0 + N
iA1 := iF0 + N
iB1 := iA1 + 2*N - 3

// Use "organ-pipe" ordering. E.g. for N=8:
// R4 F4 R3 F3 R5 F5 R2 F2 R6 F6 R1 F1 R7 F7 R0 F0
// almost half for large N; see Knuth's paper for details.
mid := N / 2
for off := 0; off <= N-mid; off++ {
i := mid - off
if i >= 0 {
s.renumBack[iR0+i] = len(s.renumFwd)
s.renumBack[iF0+i] = len(s.renumFwd) + 1
s.renumFwd = append(s.renumFwd, iR0+i, iF0+i)
}
if i = mid + off; off != 0 && i < N {
s.renumBack[iR0+i] = len(s.renumFwd)
s.renumBack[iF0+i] = len(s.renumFwd) + 1
s.renumFwd = append(s.renumFwd, iR0+i, iF0+i)
}
}

// TODO: pre-eliminate symetrical possibilities.
cols := make([]int, 4)
for i := 0; i < N; i++ {
for j := 0; j < N; j++ {
cols = iR0 + i    // Ri,         rank i
cols = iF0 + j    // Fj,         file j
a := (i + j)         // A(i+j),     diagonals
b := (N - 1 - i + j) // B(N-1-i+j), reverse diagonals
cols = cols[:2]
// Do organ-pipe reordering for R and F.
for i, c := range cols {
cols[i] = s.renumBack[c]
}

// Only add diagonals with more than one space; that
// is we omit the corners: A0, A(2N-2), B0, and B(2N-2)
if 0 < a && a < 2*N-2 {
cols = append(cols, iA1+a-1)
}
if 0 < b && b < 2*N-2 {
cols = append(cols, iB1+b-1)
}

}
}

// Search for solutions.
start := time.Now()
err := m.Search(s.found)
if err != nil {
return err
}
elapsed := time.Since(start)
fmt.Printf("%d×%d queens has %2d solutions, found in %v\n", N, N, s.count, elapsed)
if profile {
m.ProfileWrite(os.Stderr)
}
return nil
}

type solution struct {
N          int
count      int
renumFwd   []int // for "organ-pipe" column ordering
renumBack  []int
printFirst bool
}

func (s *solution) found(m *dlx.Matrix) error {
s.count++
if s.printFirst && s.count == 1 {
fmt.Printf("First %d×%d queens solution:\n", s.N, s.N)
for _, cols := range m.SolutionIDs(nil) {
var r, f int
for _, c := range cols {
// Undo organ-pipe reodering
if c < len(s.renumFwd) {
c = s.renumFwd[c]
}
if c < s.N {
r = c + 1
} else if c < 2*s.N {
f = c - s.N + 1
}
}
fmt.Printf("    R%d F%d\n", r, f)
}
}
return nil
}
```
Output:
```2×2 queens has  0 solutions, found in 1.915µs
3×3 queens has  0 solutions, found in 1.22µs
4×4 queens has  2 solutions, found in 3.095µs
5×5 queens has 10 solutions, found in 7.15µs
6×6 queens has  4 solutions, found in 17.663µs
7×7 queens has 40 solutions, found in 54.08µs
First 8×8 queens solution:
R5 F1
R1 F4
R3 F5
R4 F3
R6 F6
R2 F7
R7 F8
R8 F2
8×8 queens has 92 solutions, found in 186.991µs
9×9 queens has 352 solutions, found in 580.225µs
10×10 queens has 724 solutions, found in 2.078235ms
11×11 queens has 2680 solutions, found in 8.186708ms
12×12 queens has 14200 solutions, found in 38.037841ms
13×13 queens has 73712 solutions, found in 183.846653ms
14×14 queens has 365596 solutions, found in 961.249859ms
15×15 queens has 2279184 solutions, found in 5.491853276s
16×16 queens has 14772512 solutions, found in 33.286561009s
17×17 queens has 95815104 solutions, found in 3m34.643824374s
18×18 queens has 666090624 solutions, found in 24m22.30241617s
```

## Groovy

### Distinct Solutions

This solver starts with the N! distinct solutions to the N-Rooks problem and then keeps only the candidates in which all Queens are mutually diagonal-safe.

```def listOrder = { a, b ->
def k = [a.size(), b.size()].min()
def i = (0..<k).find { a[it] != b[it] }
(i != null) ? a[i] <=> b[i] : a.size() <=> b.size()
}

def orderedPermutations = { list ->
def n = list.size()
(0..<n).permutations().sort(listOrder)
}

def diagonalSafe = { list ->
def n = list.size()
n == 1 || (0..<(n-1)).every{ i ->
((i+1)..<n).every{ j ->
!([list[i]+j-i, list[i]+i-j].contains(list[j]))
}
}
}

def queensDistinctSolutions = { n ->
// each permutation is an N-Rooks solution
orderedPermutations((0..<n)).findAll (diagonalSafe)
}
```

### Unique Solutions

Unique solutions are equivalence classes of distinct solutions, factoring out all reflections and rotations of a given solution. See the Wikipedia page for more details.

```class Reflect {
public static final diag = { list ->
final n = list.size()
def tList =  * n
(0..<n).each { tList[list[it]] = it }
tList
}

public static final vert = { list ->
list.reverse()
}

public static final horiz = { list ->
final n = list.size()
list.collect { n - it - 1 }
}
}

enum Rotations {
r0([]),
r90([Reflect.vert, Reflect.diag]),
r180([Reflect.vert, Reflect.diag, Reflect.vert, Reflect.diag]),
r270([Reflect.diag, Reflect.vert]);

private final List operations

private Rotations(List ops) {
operations = ops ?: []
}

public static void eliminateDups(primary, solutions) {
(r0..r270).each { rot -> rot.eliminateDuplicates(primary, solutions) }
}

private void eliminateDuplicates(primary, solutions) {
def rotated = [] + primary
operations.each { rotated = it(rotated) }
solutions.removeAll([rotated, Reflect.vert(rotated)])
}
}

def queensUniqueSolutions = { start ->
assert start instanceof Number || start instanceof List
def qus = (start instanceof Number) \
? queensDistinctSolutions(start) \
: [] + start
for (def i = 0; i < qus.size()-1; i++) {
Rotations.eliminateDups(qus[i], qus[(i+1)..<(qus.size())])
}
qus
}
```

### Test and Results

This script tests both distinct and unique solution lists.

```(1..9).each { n ->
def qds = queensDistinctSolutions(n)
def qus = queensUniqueSolutions(qds)
println ([boardSize:n, "number of distinct solutions":qds.size(), "number of unique solutions":qus.size()])
if(n < 9) { qus.each { println it } }
else { println "first:\${qus}"; println "last:\${qus[-1]}" }
println()
}
```

Interpreting the Results:

Each individual result is given as a list of N numbers. Each number represents a column number within the list-indexed row. So, the following 4-queens solution:

`[1, 3, 0, 2]`

should be interpreted as follows:

```row 0 has a queen in column 1
row 1 has a queen in column 3
row 2 has a queen in column 0
row 3 has a queen in column 2```

In other words, this:

```   ---- ---- ---- ----
3 |    |////| ♛ |////|
---- ---- ---- ----
2 |/♛/|    |////|    |
---- ---- ---- ----
1 |    |////|    |/♛/|
---- ---- ---- ----
0 |////| ♛ |////|    |
---- ---- ---- ----
0    1    2    3```

Results:

```[boardSize:1, number of distinct solutions:1, number of unique solutions:1]


[boardSize:2, number of distinct solutions:0, number of unique solutions:0]

[boardSize:3, number of distinct solutions:0, number of unique solutions:0]

[boardSize:4, number of distinct solutions:2, number of unique solutions:1]
[1, 3, 0, 2]

[boardSize:5, number of distinct solutions:10, number of unique solutions:2]
[0, 2, 4, 1, 3]
[1, 4, 2, 0, 3]

[boardSize:6, number of distinct solutions:4, number of unique solutions:1]
[1, 3, 5, 0, 2, 4]

[boardSize:7, number of distinct solutions:40, number of unique solutions:6]
[0, 2, 4, 6, 1, 3, 5]
[0, 3, 6, 2, 5, 1, 4]
[1, 3, 0, 6, 4, 2, 5]
[1, 4, 0, 3, 6, 2, 5]
[1, 4, 6, 3, 0, 2, 5]
[1, 5, 2, 6, 3, 0, 4]

[boardSize:8, number of distinct solutions:92, number of unique solutions:12]
[0, 4, 7, 5, 2, 6, 1, 3]
[0, 5, 7, 2, 6, 3, 1, 4]
[1, 3, 5, 7, 2, 0, 6, 4]
[1, 4, 6, 0, 2, 7, 5, 3]
[1, 4, 6, 3, 0, 7, 5, 2]
[1, 5, 0, 6, 3, 7, 2, 4]
[1, 5, 7, 2, 0, 3, 6, 4]
[1, 6, 2, 5, 7, 4, 0, 3]
[1, 6, 4, 7, 0, 3, 5, 2]
[2, 4, 1, 7, 0, 6, 3, 5]
[2, 4, 7, 3, 0, 6, 1, 5]
[2, 5, 1, 4, 7, 0, 6, 3]

[boardSize:9, number of distinct solutions:352, number of unique solutions:46]
first:[0, 2, 5, 7, 1, 3, 8, 6, 4]
last:[3, 1, 6, 8, 0, 7, 4, 2, 5]```

```import Control.Monad
import Data.List

-- given n, "queens n" solves the n-queens problem, returning a list of all the
-- safe arrangements. each solution is a list of the columns where the queens are
-- located for each row
queens :: Int -> [[Int]]
queens n = map fst \$ foldM oneMoreQueen ([],[1..n]) [1..n]  where

-- foldM :: (Monad m) => (a -> b -> m a) -> a -> [b] -> m a
-- foldM folds (from left to right) in the list monad, which is convenient for
-- "nondeterminstically" finding "all possible solutions" of something. the
-- initial value [] corresponds to the only safe arrangement of queens in 0 rows

-- given a safe arrangement y of queens in the first i rows, and a list of
-- possible choices, "oneMoreQueen y _" returns a list of all the safe
-- arrangements of queens in the first (i+1) rows along with remaining choices
oneMoreQueen (y,d) _ = [(x:y, delete x d) | x <- d, safe x]  where

-- "safe x" tests whether a queen at column x is safe from previous queens
safe x = and [x /= c + n && x /= c - n | (n,c) <- zip [1..] y]

-- prints what the board looks like for a solution; with an extra newline
printSolution y = do
let n = length y
mapM_ (\x -> putStrLn [if z == x then 'Q' else '.' | z <- [1..n]]) y
putStrLn ""

-- prints all the solutions for 6 queens
main = mapM_ printSolution \$ queens 6
```

If you just want one solution, simply take the `head` of the result of `queens n`; since Haskell is lazy, it will only do as much work as needed to find one solution and stop.

### Alternative version

```import Control.Monad (foldM)
import Data.List ((\\))

main :: IO ()
main = mapM_ print \$ queens 8

queens :: Int -> [[Int]]
queens n = foldM f [] [1..n]
where
f qs _ = [q:qs | q <- [1..n] \\ qs, q `notDiag` qs]
q `notDiag` qs = and [abs (q - qi) /= i | (qi,i) <- qs `zip` [1..]]
```

### Using permutations

This version uses permutations to generate unique horizontal and vertical position for each queen. Thus, we only need to check diagonals. However, it is less efficient than the previous version because it does not prune out prefixes that are found to be unsuitable.

```import Data.List (nub, permutations)

-- checks if queens are on the same diagonal
-- with [0..] we place each queen on her own row
check f = length . nub . zipWith f [0..]

-- filters out results where 2 or more queens are on the same diagonal
-- with [0..n-1] we place each queeen on her own column
generate n = filter (\x -> check (+) x == n && check (-) x == n) \$ permutations [0..n-1]

-- 8 is for "8 queens"
main = print \$ generate 8
```

### In terms of foldr

A back-tracking variant using the Prelude's plain foldr:

Translation of: JavaScript
```import Data.List (intercalate, transpose)

--------------------- N QUEENS PROBLEM -------------------

queenPuzzle :: Int -> Int -> [[Int]]
queenPuzzle nRows nCols
| nRows <= 0 = [[]]
| otherwise =
foldr
(\x y -> y <> foldr (go x) [] [1 .. nCols])
[]
\$ queenPuzzle (pred nRows) nCols
where
go qs iCol b
| safe (nRows - 1) iCol qs = b <> [qs <> [iCol]]
| otherwise = b

safe :: Int -> Int -> [Int] -> Bool
safe iRow iCol qs =
(not . or) \$
zipWith
( \sc sr ->
(iCol == sc) || (sc + sr == (iCol + iRow))
|| (sc - sr == (iCol - iRow))
)
qs
[0 .. iRow - 1]

--------------------------- TEST -------------------------
-- 10 columns of solutions for the 7*7 board:
showSolutions :: Int -> Int -> [String]
showSolutions nCols nSize =
unlines
. fmap (intercalate "   ")
. transpose
. map boardLines
<\$> chunksOf nCols (queenPuzzle nSize nSize)
where
go r x
| r == x = '♛'
| otherwise = '.'
boardLines rows =
[ go r <\$> [1 .. (length rows)]
| r <- rows
]

chunksOf :: Int -> [a] -> [[a]]
chunksOf i = splits
where
splits [] = []
splits l = take i l : splits (drop i l)

main :: IO ()
main = (putStrLn . unlines) \$ showSolutions 10 7
```
Output:
```......♛   ......♛   ......♛   ......♛   .....♛.   .....♛.   .....♛.   .....♛.   .....♛.   .....♛.
.♛.....   ..♛....   ...♛...   ....♛..   ♛......   .♛.....   ..♛....   ..♛....   ..♛....   ...♛...
...♛...   .....♛.   ♛......   ..♛....   ..♛....   ....♛..   ......♛   ....♛..   ♛......   ......♛
.....♛.   .♛.....   ....♛..   ♛......   ....♛..   ♛......   ...♛...   ......♛   ...♛...   ♛......
♛......   ....♛..   .♛.....   .....♛.   ......♛   ...♛...   ♛......   ♛......   ......♛   ..♛....
..♛....   ♛......   .....♛.   ...♛...   .♛.....   ......♛   ....♛..   ...♛...   ....♛..   ....♛..
....♛..   ...♛...   ..♛....   .♛.....   ...♛...   ..♛....   .♛.....   .♛.....   .♛.....   .♛.....

.....♛.   ....♛..   ....♛..   ....♛..   ....♛..   ....♛..   ....♛..   ...♛...   ...♛...   ...♛...
...♛...   ♛......   ♛......   .♛.....   ..♛....   ......♛   ......♛   ♛......   ♛......   .♛.....
.♛.....   .....♛.   ...♛...   .....♛.   ♛......   .♛.....   .♛.....   ....♛..   ..♛....   ......♛
......♛   ...♛...   ......♛   ..♛....   .....♛.   ...♛...   .....♛.   .♛.....   .....♛.   ....♛..
....♛..   .♛.....   ..♛....   ......♛   ...♛...   .....♛.   ..♛....   .....♛.   .♛.....   ..♛....
..♛....   ......♛   .....♛.   ...♛...   .♛.....   ♛......   ♛......   ..♛....   ......♛   ♛......
♛......   ..♛....   .♛.....   ♛......   ......♛   ..♛....   ...♛...   ......♛   ....♛..   .....♛.

...♛...   ...♛...   ...♛...   ..♛....   ..♛....   ..♛....   ..♛....   ..♛....   ..♛....   .♛.....
.....♛.   ......♛   ......♛   ♛......   ♛......   ....♛..   .....♛.   ......♛   ......♛   ...♛...
♛......   ....♛..   ..♛....   .....♛.   .....♛.   ......♛   .♛.....   ...♛...   .♛.....   .....♛.
..♛....   .♛.....   .....♛.   .♛.....   ...♛...   .♛.....   ....♛..   ♛......   ...♛...   ♛......
....♛..   .....♛.   .♛.....   ....♛..   .♛.....   ...♛...   ♛......   ....♛..   .....♛.   ..♛....
......♛   ♛......   ....♛..   ......♛   ......♛   .....♛.   ...♛...   .♛.....   ♛......   ....♛..
.♛.....```