051 - N-Queens

The n-queens puzzle is the problem of placing n queens on an n×n chessboard such that no two queens attack each other.

Given an integer n, return all distinct solutions to the n-queens puzzle.

Each solution contains a distinct board configuration of the n-queens' placement, where 'Q' and '.' both indicate a queen and an empty space respectively.

For example,
There exist two distinct solutions to the 4-queens puzzle:

[
 [".Q..",  // Solution 1
  "...Q",
  "Q...",
  "..Q."],

 ["..Q.",  // Solution 2
  "Q...",
  "...Q",
  ".Q.."]
]

static int Q; 
static char ***ret;
static int retsize;
static int retcur;
void printq(char **queen)
{
	int i,j;
	if (retcur >= retsize) {
		char ***tmp = malloc(sizeof(char **) * 2 * retsize);
		for (i = 0; i < retsize; i++)
			tmp[i] = ret[i];
		free(ret);
		ret = tmp;
		//ret = realloc(ret, retsize * 2);
		for (i = retsize; i < 2 * retsize; i++) {
			ret[i] = (char **)malloc(sizeof(char *) * Q);
			for (j = 0; j < Q; j++) {
				ret[i][j] = (char *)malloc(sizeof(char) * (Q+1));
				ret[i][j][Q] = '\0';
			}
		}
		retsize *= 2;
	}
	
	for(i = 0; i< Q; i++) 
		memcpy(ret[retcur][i], queen[i], Q);
	retcur ++;
}
int locok(char **queen, int cur, int b)
{
	int i,j;
	for(i = 0; i< cur; i++)
	    for(j = 0; j < Q; j++) {
		if (queen[i][j] == 'Q') {
			if (i == cur || j == b) return 0;
			if (abs(i - cur) == abs(j - b)) return 0;
		}
	    }
	return 1;
}

void x8q(char **queen, int cur)
{
	if (cur == Q) return;
	if (cur == Q - 1) {
		int i;
		for(i = 0; i < Q;i ++) {
			if (locok(queen, cur, i)) {
				queen[cur][i] = 'Q';
				printq(queen);
				queen[cur][i] = '.';
			}
		}
	} else {
		int i;
		for(i = 0; i < Q; i ++) {
			if (locok(queen, cur, i)) {
				queen[cur][i] = 'Q';
			        x8q(queen, cur + 1);
				queen[cur][i] = '.';
			}	
		}
	}
}

char*** solveNQueens(int n, int* returnSize) 
{
	int i, j;
	retcur = 0;
	retsize = 8192;
	ret = malloc(sizeof(char **) * retsize);
	for (i = 0; i < retsize; i++) {
		ret[i] = (char **)malloc(sizeof(char *) * n);
		for (j = 0; j < n; j++) {
			ret[i][j] = (char *)malloc(sizeof(char) * (n + 1));
			ret[i][j][n] = '\0';
		}
	}
	char **queen = (char **)malloc(sizeof(char *) * n);
	for (i = 0; i < n; i++) {
		queen[i] = (char *)malloc(sizeof(char) * n);
		memset(queen[i], '.', n);
	}

	Q = n;
	x8q(queen, 0);
	*returnSize = retcur;
	return ret;
}