backtracking_8queen.c

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

int board[8][8];

int check(int x, int y) {
    int p, q;

    for (p = 0; p < x; p++) {
        if (board[p][y] == 1)
            return 0;
    }

    p = x; q = y;
    while ( p > 0 && q > 0 ) {
        if (board[--p][--q] == 1)
            return 0;
    }

    p = x; q = y;
    while ( p > 0 && q < 7 ) {
        if (board[--p][++q] == 1)
            return 0;
    }

    return 1;
}

void showboard(void)
{
    int x, y;
    for ( y = 0; y < 8; y++ ) {
        for ( x = 0; x < 8; x++ ) {
            printf("%c ", board[x][y] ? 'Q' : '.');
        }
        printf("\n");
    }
}

int solve(int x)
{
    int i;

    if (x == 8)
        return 1;

    for (i = 0; i < 8; i++) {
        if (check(x, i)) {
            board[x][i] = 1;

            if (solve(x + 1))
                return 1;

            board[x][i] = 0;
        }
    }

    return 0;
}

int main(void)
{
    if (solve(0)) {
        showboard();
    }

    return EXIT_SUCCESS;
}