BFS板子死活没找出错来

#include<iostream>
#include<cstdio>
#include<algorithm>
#include<cstring>
#include<queue>
using namespace std;
int n;
int xx[4]={0,0,1,-1};
int yy[4]={1,-1,0,0};
int dis[1010][1010],vis[1010][1010];
char f[1010][1010];
struct node{
    int a,b;
};
queue<node> q;
int x1,y1,fx,fy,sx,sy;
int bfs(int x,int y)
{
    q.push((node){x,y});
    vis[x][y]=1;
    while(!q.empty());
    {
        x1=q.front().a;
        y1=q.front().b;
        q.pop();
        if(x1==fx&&y1==fy)
        {
            return dis[x1][y1];
        }
        for(int i=0;i<4;i++)
        {
            int dx=x1+xx[i];
            int dy=y1+yy[i];
            if(dx>0&&dx<=n&&dy>0&&dy<=n&&vis[dx][dy]==0&&f[dx][dy]=='0')
            {
                vis[dx][dy]=1;
                dis[dx][dy]=dis[x1][y1]+1;
                q.push((node){dx,dy});
            }
        }
    }
}
char d;
int main()
{
    cin>>n;
    for(int i=1;i<=n;i++)
    {
        for(int j=1;j<=n;j++)
        {
            cin>>f[i][j];
        }
    }
    cin>>sx>>sy>>fx>>fy;
    cout<<bfs(sx,sy);
    return 0;
}