N-queens problem: Difference between revisions
→{{header|Pascal}}
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=={{header|Pascal}}==
A basic recursive backtracking version taken from the book ''Algorithms + Data Structures = Programs'', with comments a few added.
<syntaxhighlight lang="pascal">
program eightqueens(output);
var i: integer;
a: array [1..8] of boolean; { a[j]: no queen in row j }
b: array [2..16] of boolean; { b[k]: no queen in kth diagonal down-left }
c: array [-7..7] of boolean; { c[k]: no queen in kth diagonal down-right }
x: array [1..8] of integer; { x[i]: position of queen in column i }
procedure print;
var k: integer;
begin
for k := 1 to 8 do write(x[k]: 4);
writeln
end { print } ;
procedure try(i: integer);
var j: integer;
begin
for j := 1 to 8 do
if a[j] and b[i+j] and c[i-j] then
begin
{ place queen }
x[i] := j;
a[j] := false; b[i+j] := false; c[i-j] := false;
if i < 8 then try(i+1) else print;
{ remove queen }
a[j] := true; b[i+j] := true; c[i-j] := true
end
end { try } ;
begin
for i := 1 to 8 do a[i] := true;
for i := 2 to 16 do b[i] := true;
for i := -7 to 7 do c[i] := true;
try(1)
end .</syntaxhighlight>
<syntaxhighlight lang="pascal">program queens;
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