为什么机子上测测试数据和答案一样还WA

#include <bits/stdc++.h>
using namespace std;
char a[1005][1005];
int ans[1005][1005],mp[1005][1005],n;
bool vis[1005][1005];
struct Node{
	int x,y;
}begin,end,pos;
int dx[]={-1,0,1,0};
int dy[]={0,1,0,-1};
queue<Node> q;
int bfs()
{
	vis[begin.x][begin.y]=1;
	q.push(begin);
	while (!q.empty())
	{
		pos=q.front();
		q.pop();
		for (int i=0;i<4;++i)
		{
			int xx=pos.x+dx[i];
			int yy=pos.y+dy[i];
			if (xx>=1&&xx<=n&&yy>=1&&yy<=n&&vis[xx][yy]==0&&mp[xx][yy]==0)
			{
				q.push({xx,yy});
				vis[xx][yy]=1;
				ans[xx][yy]=ans[pos.x][pos.y]+1;
			}
		}
		if (vis[end.x][end.y]) return ans[end.x][end.y];
	}
	return ans[end.x][end.y]?ans[end.x][end.y]:-1;
}
int main()
{
	scanf ("%d",&n);
	for (int i=1;i<=n;++i)
	{
		for (int j=1;j<=n;++j)
		{
			scanf ("%c",&a[i][j]);
			mp[i][j]=a[i][j]-'0';
		}
		getchar();
	}	
	scanf ("%d%d%d%d",&begin.x,&begin.y,&end.x,&end.y);
	printf ("%d\n",bfs());
	return 0;
}