#5wa了

92

代码如下：

#include <iostream>
using namespace std;
const int NR=2501;
int n,m,i,j,a[NR][NR],leftt[NR][NR],up[NR][NR],f[NR][NR],answer,ans,ans1,ans2;
void fan()
{
	for(i=1;i<=(n+1)/2;i++)
		for(j=1;j<=(m+1)/2;j++)
		{
			swap(a[i][j],a[i][m-j+1]);
		}
}
int solve()
{
	ans=-1;
	//leftt
	for(i=1;i<=n;i++)
		for(j=1;j<=m;j++)
		{
			if(a[i][j]==0)
				leftt[i][j]=leftt[i][j-1]+1;
			else
				leftt[i][j]=0;
		}
	//up
	for(j=1;j<=m;j++)
		for(i=1;i<=n;i++)
		{
			if(a[i][j]==0)
				up[i][j]=up[i-1][j]+1;
			else
				up[i][j]=0;
		}
	//f
	for(i=1;i<=n;i++)
		for(j=1;j<=m;j++)
		{
			if(a[i][j]==0)
				f[i][j]=0;
			else
			{
				f[i][j]=min(up[i-1][j],min(leftt[i][j-1],f[i-1][j-1]))+1;
			}
			ans=max(f[i][j],ans);
		}
		return ans;
}
int main()
{
	ans=-1;
	//input
	cin>>n>>m;
	for(i=1;i<=n;i++)
	{
		for(j=1;j<=m;j++)
			cin>>a[i][j];
	}
	ans1=solve();
	fan();
	ans2=solve();
	cout<<max(ans1,ans2)<<endl;	
	return 0;
}

分两次DP，分别取左下和右下开吃。

蒟蒻求调！！！