nqueens

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

int _doit(int n, int c, int *h)
{
    int count;
    int i;
    int j;

    count = (i = -1);
    while ((c - n) && (j = !(++i < n) -1)) {
        while (++j < c && !(h[j] == i || abs(h[j] - i) == c - j));
        if (j < c) continue;
        h[c] = i;
        count += _doit(n, c + 1, h);
    }
    return (++count + !(c - n));
}

int count_valid_queen_placement(int n) {
    int h[n];

    return _doit(n, 0, h);
}

int main(int n, char **argv)
{
    if (n <= 1 || (n = atoi(argv[1])) <= 0) n = 8;
    printf("%d\n", count_valid_queen_placement(n));
}