求助帖有大佬帮忙看看哪里出问题了么QAQ

#include<iostream>
#include<algorithm>
#include<cstring>
#include<queue>

#define x first
#define y second

using namespace std;
typedef pair<int, int> PII; //存坐标

const int N = 1010;
PII source, terminal;
int n,g[N][N];      //存地图
int dist[N][N];     //存从发点到该坐标的距离
queue <PII> q;

int dx[] = {-1, 0, 1, 0};
int dy[] = {0, 1, 0, -1};

int bfs(PII s){
    memset(dist,-1,sizeof dist);        //将距离矩阵初始化为-1
    dist[s.x][s.y] = 0;                 //发点距离设为0
    q.push(s);
    while(!q.empty()){
        PII cur_cdn = q.front();
        for(int i = 0; i < 4; i++){ 
            int a = cur_cdn.x + dx[i], b = cur_cdn.y + dy[i];
            if(a < 1 || a > n || b < 1 || b > n) continue;
            if(g[a][b] == 1) continue;
            if(dist[a][b] != -1) continue;
            q.push({a,b});
            dist[a][b] = dist[cur_cdn.x][cur_cdn.y] + 1;

        }
        q.pop();
    }
    return dist[terminal.x][terminal.y];
}

int main(){
    //数据输入
    cin >> n;
    cin.ignore();
    for (int i = 1; i <= n; i++) {
        string line;
        getline(cin, line);
        // 将字符串中的每个字符转换为整数存储到二维数组中
        for (int j = 1; j <= n; j++) {
            g[i][j] = line[j-1] - '0';  // 将字符转换为整数
        }
    }
    cin >> source.x >> source.y >> terminal.x >> terminal.y;

    //开始BFS
    int res = bfs(source);
    cout << res << endl;
}