http://acm.hdu.edu.cn/showproblem.php?pid=4324

只要有三点以上可以成环即可，自己想想为什么，用到拓扑排序，是否成环

　　

#include <cstdio>
#include <stack>
#include <cstring>
using namespace std;
char s[2001][2001];
int ingree[2001],n;
stack<int> v;
int solve()
{
    while(!v.empty()) v.pop();
    for(int i = 1;i <= n;i ++)
    if(ingree[i] == 0)
    v.push(i);
    int count = 0,j;
    while(!v.empty())
    {
        j = v.top();
        v.pop();
        count ++;
        for(int i = 1;i <= n;i ++)
        {
            if(s[j - 1][i - 1] == '1' && ingree[i] > 0)
            {
                ingree[i] --;
                if(ingree[i] == 0)
                v.push(i);
            }
        }
    }
    if(count < n) return false;
    return true;
}
int main()
{
    int t,u = 0;
    scanf("%d",&t);
    while(t --)
    {
        scanf("%d",&n);
        memset(ingree,0,sizeof(ingree));
        for(int i = 0;i < n;i ++)
        {
            scanf("%s",s[i]);
            for(int j = 0;j < n;j ++)
            if(s[i][j] == '1')
            ingree[j + 1] ++;
        }
        if(solve())
        printf("Case #%d: No\n",++ u);
        else
        printf("Case #%d: Yes\n",++ u);
    }
    return 0;
}