求助！求助！求助！

样例过了， 但是提交0分，实在不知道错哪里了

#include<iostream>
#include<cstdio>
#include<queue>

using namespace std;
const int N = 1e4 + 5;

int n, f[N][N], map[N][N];
int dx[4] = {1, -1, 0, 0};
int dy[4] = {0, 0, 1, -1};
bool book[N][N];
char ab[N];

void bfs(int a, int b){
    queue <int> qx;
    queue <int> qy;
    qx.push(a);
    qy.push(b);
    book[a][b] = 1;
    f[a][b] = 0;
    while(!qy.empty()){
        int xx = qx.front();
        int yy = qy.front();
        qx.pop(); qy.pop();
        for(int i=0; i<4; ++i){
            int x = xx + dx[i];
            int y = yy + dy[i];
            if(x>=1&&x<=n&&y>=1&&y<=n&&!book[x][y]&&!map[x][y]){
                qx.push(x);
                qy.push(y);
                f[x][y] = f[xx][yy] + 1;
                book[x][y] = 1;
            }
        }
    }
}

int main(){
    int x, y, xx, yy;
    scanf("%d", &n);
    for(int i=1; i<=n; ++i){
        scanf("%s", ab+1);
        for(int j=1; j<=n; ++j){
            map[i][j] = ab[i] - '0';
        }
    }  
    scanf("%d%d%d%d", &x, &y, &xx, &yy);
    bfs(x, y);
    printf("%d\n", f[xx][yy]);
    return 0;
}