HDU 3342

//题目类型：拓扑排序
//解题思路：利用拓扑排序，如果在一次排序中，所有点的入度均不为0，则判断存在环，即非法。
#include <iostream>
//#include <conio.h>
using namespace std;
#define narray 101
int graph[narray][narray];
int indegree[narray];
int n,m;
int main()
{
    //freopen("1.txt","r",stdin);
    int i,j,k;
    int p1,p2;
    bool flag;
    while(scanf("%d%d",&n,&m)!=-1)
    {
        if(n==0) break;
        memset(indegree,0,sizeof(indegree));
        memset(graph,0,sizeof(graph));
        for(i=1;i<=m;++i)
        {
            scanf("%d%d",&p1,&p2);
            if(!graph[p1][p2])
            {
                graph[p1][p2]=1;
                indegree[p2]++;
            }
        }
        for(i=1;i<=n;++i)
        {
            flag = false;      //用于标识在一次排序中是否有环
            for(j=0;j<n;++j)
            {
                if(indegree[j]==0)
                {
                    flag = true;
                    indegree[j]--;
                    for(k=0;k<n;++k)
                    {
                        if(graph[j][k])
                          indegree[k]--;
                    }
                    break;
                }
            }
            if(!flag)
            {
                printf("NO\n");
                break;
            }
        }
        if(i==n+1) printf("YES\n");
    }
    //getch();
    return 0;
}