BFS与MLE求助

#include <bits/stdc++.h>
using namespace std;
struct f{
    int x,y;
}qi,zhong;
int n,dis[1005][1005];
char s;
bool vis[1005][1005];
queue<f>q;
int dx[4]={1,0,0,-1};
int dy[4]={0,1,-1,0};
void bfs(){
    while(!q.empty()){
        f now=q.front(); q.pop();
        vis[now.x][now.y]=true;
        for(int i=0;i<4;i++){
            int xx=now.x+dx[i],yy=now.y+dy[i];
            if(xx<=0 || xx>n || yy<=0 || yy>n) continue;
            if(vis[xx][yy]==false){
                dis[xx][yy]=dis[now.x][now.y]+1;
                if(xx==zhong.x && yy==zhong.y) return;
                q.push({xx,yy});
            }
        }
    }
}
int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    cin>>n;
    for(int i=1;i<=n;i++){
        for(int j=1;j<=n;j++){
            cin>>s;
            if(s=='1') vis[i][j]=true;
        }
    }
    cin>>qi.x>>qi.y>>zhong.x>>zhong.y;
    q.push({qi.x,qi.y});
    vis[qi.x][qi.y]=true;
    bfs();
    cout<<dis[zhong.x][zhong.y];
    return 0;
}