坑啊 有不有！一个点不能通过是什么意思！？？？？

题目坑！

#include<iostream>
#include<cstdio>
#include<cstring>
using namespace std;
bool  c[1001][1001]={0};
long pre[1000001];
struct s
{
    long x,y;
}s[1000001];
long n,m;
int dx[4]={0,0,1,-1};
int dy[4]={1,-1,0,0};
long ans=0;
void  liu(int h)
{
    ans++;
    if(pre[h]!=1)
    liu(pre[h]);
}
int main()
{
    long i,j,head,tail,x0,y0,x,y;
    bool f=0;
    string r[1001];
    scanf("%ld",&n);
    for(i=1;i<=n;i++)
    cin>>r[i];
    m=r[1].length();
    cin>>x0>>y0>>x>>y;
      head=0;tail=1;
      s[1].x=x0;
      s[1].y=y0;
      pre[1]=0;
      c[x0][y0]=1;
      while(head<tail)
      {
          head++;
          for(int o=0;o<4;o++)
           {
               int a=s[head].x+dx[o];
               int b=s[head].y+dy[o];
               if(a>0&&a<=n&&b>0&&b<=m&&c[a][b]==0&&r[a][b]=='0')
               {
                   tail++;
                   s[tail].x=a;
                   s[tail].y=b;
                   pre[tail]=head;
                   c[a][b]=1;
            }
            if(a==x&&b==y)
             {
                 f=1;
                 break;
             }
         }
         if(f==1)  break;
      }
      liu(tail);
     cout<<ans;
}