N-queens problem: Difference between revisions

The algorithm can be slightly improved by using sets instead of lists. But this makes the algorithm a bit harder to read, since the list x has to be added to record the solution. On a regular 8x8 board only 5,508 possible queen positions are examined.

<syntaxhighlight lang="python">def queens(i: int, a: set, b: set, c: set, x: list):

if a: # set a is not empty

for j in a:

if i + j not in b and i - j not in c:

yield from queens(i + 1, a - {j}, b | {i + j}, c | {i - j}, x + [j])

else:

yield x

for solution in queens(0, set(range(8)), set(), set(), []):

print(solution)</syntaxhighlight>

However, because these examples are intended for educational purposes, they are not optimized for resources or speed. The next example shows a more optimized solution that also requires less space on the stack.