Here is a depth first search solution that was posted to comp.lang.awk group some years ago.

The original posting is on my scratchpad. All that I've done is to modify the output of a2p on the original awk code.

Coincidentally, the usenet post was comparing the speed of awk and Perl and a prominent Perlmonk gets a mention. The author of the awk program is Mike Brennan who wrote mawk. Ultimately however, these lang versus lang games prove very little.

#!/usr/local/bin/perl -w

# Comments from the origianl posting
#
#    queens    # solves 8-queen problem
#    queens 5  # solves 5-queen problem
#    1  3  5  2  4
#    5  3  1  4  2
#    1  4  2  5  3
#    5  2  4  1  3
#    2  4  1  3  5
#    4  2  5  3  1
#    2  5  3  1  4
#    4  1  3  5  2
#
# Each row shows a solution and the 5-queens problem has 8 solutions.
# The first solution (1 3 5 2 4) is
#
#    Q * * * *
#    * * Q * *
#    * * * * Q
#    * Q * * *
#    * * * Q *
#

use strict;

my $sz = $ARGV[0] || 8;

die "Board must be >= 4x4\n" if $sz < 4;

my @soln;

for (my $i = 1; $i <= $sz / 2; $i++) {
    $soln[0] = $i;

    df_queens($sz, 1, @soln);
}


# print a solution and it's symmetry about the vertical axis
sub print_soln {

    my $soln_sz =  shift;
    my @soln    = @_;

    printf ' %2d', $_ for @soln;
    print "\n";

    # print symmetric soln
    my $X = $soln_sz + 1;
    printf ' %2d', $X - $_ for @soln;
    print "\n";
}


# depth first seach for solutions
sub df_queens {

    my $prob_sz = shift;
    my $soln_sz = shift;
    my @soln    = @_;

    if ($prob_sz == $soln_sz) {
        print_soln($soln_sz, @soln );
        return;
    }

    my $new_r   = $soln_sz + 1;

    for my $new_c ( 1 .. $prob_sz) {
        my $ok = 1;
        for my $r   ( 1 .. $soln_sz) {
            my $c = $soln[$r-1];
            if ( $c       == $new_c             ||
                ($c + $r) == ($new_c + $new_r)  ||
                ($c - $r) == ($new_c - $new_r))
            {
                $ok = 0;
                last;
            }
        }

        if ($ok) {
            $soln[$new_r-1] = $new_c;
            df_queens($prob_sz, $new_r, @soln);
        }
    }
}
[download]

Update: Fixed 2 errors as per blakem's post below.

--
John.

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