求大神指错。。

#include<iostream>
#include<cstdio>
#include<cstring>
#include<cmath>
#include<algorithm>
const int N=2510;
int f[N][N],map[N][N],l[N][N],r[N][N],n,m,maxf;
using namespace std;
int main()
{
    freopen("in.txt","r",stdin);
    freopen("out.txt","w",stdout);
    scanf("%d%d",&n,&m);
    for(int i=1;i<=n;i++)
    for(int j=1;j<=m;j++)
    scanf("%d",&map[i][j]);
    for(int i=1;i<=m;i++)
    {
        int x1=1,y1=i;
        while(x1<=n&&y1<=m)
        {
            if(map[x1][y1])
            r[x1][y1]=r[x1-1][y1-1]+1;
            x1++;y1++;
         } 
         int x2=1,y2=i;
         while(x2<=n&&y2>=1)
         {
             if(map[x2][y2])
            l[x2][y2]=l[x2-1][y2+1]+1;
            x2++;y2--;
         }
    }
    for(int i=2;i<=n;i++)
    {
        int x1=i,y1=1;
        while(x1<=n&&y1<=m)
        {
            if(map[x1][y1])
            r[x1][y1]=r[x1-1][y1-1]+1;
            x1++;y1++;
         } 
         int x2=i,y2=m;
         while(x2<=n&&y2>=1)
         {
             if(map[x2][y2])
            l[x2][y2]=l[x2-1][y2+1]+1;
            x2++;y2--;
         }
    }
    for(int i=1;i<=n;i++)
    for(int j=1;j<=m;j++)
    f[i][j]=map[i][j]+f[i-1][j]+f[i][j-1]-f[i-1][j-1];
    for(int i=1;i<=n;i++)
    for(int j=1;j<=m;j++)
    {
        if(r[i][j])
        {
            int x=r[i][j];
            if(f[i][j]-f[i-x][j]-f[i][j-x]+f[i-x][i-x]==x)
            maxf=max(x,maxf);
        }
    }
    memset(f,0,sizeof(f));
    for(int i=1;i<=n;i++)
    for(int j=m;j>=1;j--)
    f[i][j]=map[i][j]+f[i-1][j]+f[i][j+1]-f[i-1][j+1];
    for(int i=1;i<=n;i++)
    for(int j=m;j>=1;j--)
    {
        if(l[i][j])
        {
            int x=l[i][j];
            if(f[i][j]-f[i-x][j]-f[i][j+x]+f[i-x][i+x]==x)
            maxf=max(x,maxf);
        }
    }
    cout<<maxf;
    fclose(stdin);fclose(stdout);
    return 0;
}