题目给的样例都输出-1；大佬帮忙看看吧

#include<bits/stdc++.h>
#define x first
#define y second
//define后面不用分号； 
//define a b 是将b定义成a；注意顺序； 
using namespace std;
typedef pair<int,int> PII;
const int N=1010;
char G[N][N];
int dist[N][N];
int n; 
queue<PII> q;
int dx[]={0,1,0,-1};
int dy[]={1,0,-1,0};
int dfs(int x1,int y1,int x2,int y2)
{
	//忘记初始化数组
	memset(dist,-1,sizeof(dist)); 

	dist[x1][y1]=0;
	q.push({x1,y1});

	while(q.size())
	{auto t=q.front();
		q.pop();
	
		for(int i=0;i<4;i++)
		{
			
			int a=t.x+dx[i];
			int b=t.y+dy[i];
			
		 if(a<0||a>=n||b<0||b>=n)
		 continue;
		 
		 if(G[a][b]=='1')
		 continue;
		 if(dist[a][b]>=0)
		 continue;
	
		 q.push({a,b});
		 dist[a][b]=1+dist[t.x][t.y];
		 if(a==x2&&b==y2)
		 	return dist[x2][y2];
		 } 
		
	}
	return -1;
	
}
int main()
{
	cin>>n;
	int x1,x2,y1,y2;
	for(int i=0;i<n;i++)
	{
		scanf("%s",G[i]);
	}
	cin>>x1>>y1>>x2>>y2; 
	cout<<dfs(x1,y1,x2,y2);
	 
	return 0;
 }