分析：BFS判斷是否有環。 

#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
int gra[200][200];
int vis[100];
void bfs(int n)
{
    memset(vis,0,sizeof(vis));
    vis[1] = 1;
    int q[100];
    int in = 0,out = 0, f = 0;
    q[in ++] = 1;
    while(in > out)
    {
        int x = q[out ++];
        for(int i = 1; i <= n; i ++)
        {
            if(gra[x][i])
            {
                if(vis[i]){
                    f = 1;
                    break;
                }
                else {
                    vis[i] = 1;
                    q[in ++] = i;
                }
            }
        }
        if(f)break;
    }
    if(f)printf("NO\n");
    else printf("YES\n");
}
int main()
{
    int n,m,u,v;
    while(scanf("%d%d",&n,&m)!=EOF){
        memset(gra,0,sizeof(gra));
        for(int i = 0; i < m; i ++){
        scanf("%d%d",&u,&v);
        gra[u][v] = 1;
        }
        bfs(n);
    }
    return 0;
}