Given a m-by-n (0,1)-matrix, of all its submatrices of all 1’s which is the largest? By largest we mean that the submatrix has the most elements.

The input contains multiple test cases. Each test case begins with m and n (1 ≤ m, n ≤ 2000) on line. Then come the elements of a (0,1)-matrix in row-major order on m lines each with n numbers. The input ends once EOF is met.

For each test case, output one line containing the number of elements of the largest submatrix of all 1’s. If the given matrix is of all 0’s, output 0.

//
//  main.cpp
//  poj3494
//
//  Created by Candy on 10/5/16.
//  Copyright © 2016 Candy. All rights reserved.
//

#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cstring>
using namespace std;
typedef long long ll;
const int N=2e3+5;
inline int read(){char c=getchar();int x=0,f=1;while(c<'0'||c>'9'){if(c=='-')f=-1;c=getchar();}while(c>='0'&&c<='9'){x=x*10+c-'0';c=getchar();}return x;
}
int n,m,tot[N],a,ans=0;
struct data{int h,l,pos;
}st[N];
int top=0;
int main(int argc, const char * argv[]) {while(scanf("%d%d",&n,&m)!=EOF){ans=0;memset(tot,0,sizeof(tot));for(int i=1;i<=n;i++){top=0;for(int j=1;j<=m;j++){a=read();if(!a) tot[j]=-1;data t;t.h=++tot[j];t.l=1;t.pos=j;int right=0;while(top&&st[top].h>=t.h){ans=max(ans,(st[top].l+right)*st[top].h);//printf("%d %d %d  %d %d %d\n",i,j,ans,st[top].l,right,st[top].h);right+=st[top].l; t.l+=st[top].l;top--;}st[++top]=t;//printf("top %d\n",top);}//printf("toptop %d\n",top);while(top){ans=max(ans,st[top].h*(st[top].l +n-st[top].pos));//printf("p %d %d %d %d\n",st[top].h,st[top].pos,st[top].l,ans);top--;}}printf("%d\n",ans);}return 0;
}