36分，泥萌谁帮我DEBUG下谢了

#include <cstdio>

#define re register
#define max(a,b) ((a)>(b)?(a):(b))
#define min(a,b) ((a)<(b)?(a):(b))

const int N = 2500 + 10;

int n,m;
int g[N][N];
int s[N][N]; 
int l,r,mid,ans;

bool check(int x) {
    for(re int i=x; i<=n; i++) {
        for(re int j=x; j<=m; j++) {
            if(g[i][j]) {
                //printf("i=%d j=%d s=%d x=%d\n",i,j,s[i][j]-s[i-x][j]-s[i][j-x]+s[i-x][j-x],x);
                if(s[i][j]-s[i-x][j]-s[i][j-x]+s[i-x][j-x] == x) {
                    for(re int k=1; k<x; ++k) {
                        if(!g[i-k][j-k]) {
                            break;
                        }
                        if(k == x-1) {
                            return true;
                        } 
                    }
                }
                if(s[i][j]-s[i-x][j]-s[i][j+x]+s[i-x][j+x] == x) {
                    for(re int k=1; k<x; ++k) {
                        if(!g[i-k][j+k] || j+k > m) {
                            break;
                        } 
                        if(k == x-1) {
                            return true;
                        }
                    }
                }
            }
        }
    }
    return false;
}

void Input() {
    scanf("%d%d",&n,&m);
    for(re int i=1; i<=n; ++i) {
        for(re int j=1; j<=m; ++j) {
            scanf("%d",&g[i][j]);
            s[i][j]=s[i][j-1]+s[i-1][j]-s[i-1][j-1]+g[i][j];
        }
    }
}

void Solve() {
    l=1,r=min(n,m);
    while(l <= r) {
        mid=(l+r)>>1;
        if(check(mid)) {
            l=mid+1;
            ans=mid;
        } else {
            r=mid-1;
        }
    }
    printf("%d",ans);
}

int main(void) {
    Input();
    Solve();
    return 0;
}