1 #include<iostream>
 2 #include<cstdio>
 3 #include<cstring>
 4 using namespace std;
 5 #define N 150
 6 struct Edge{
 7     int to;
 8     int next;
 9 }edge[N];
10 int head[N];
11 int Enct;
12 int in[N];
13 void init()
14 {
15     Enct = 0;
16     memset(head,-1,sizeof(head));
17     memset(in,0,sizeof(in));
18 }
19 void add(int from , int to )
20 {
21     edge[Enct].to = to;
22     edge[Enct].next = head[from];
23     head[from]= Enct++;
24 }
25 int que[N];
26 int n;
27 bool ph()
28 {
29     int c = 0;
30     for(int i = 0 ; i < n ;i++)
31     {
32         if(in[i]==0) que[c++] = i;
33     }
34     for(int i = 0 ; i < c; i++)
35     {
36         for(int j = head[que[i]] ; j!=-1; j= edge[j].next)
37         {
38             Edge e = edge[j];
39             in[e.to]--;
40             if(in[e.to]==0)
41             que[c++] = e.to;
42         }
43     }
44     //printf("c = %d\n",c);
45     if(c<n-1) return false ;
46     else return true;
47 }
48 int main()
49 {
50     int m ;
51     while(~scanf("%d%d",&n,&m)&&(n!=0||m!=0))
52     {
53         init();
54         for(int i = 0 ;i < m ;i++)
55         {
56             int a , b;
57             scanf("%d%d",&a,&b);
58             add(a,b);
59             in[b]++;
60         }
61         if(ph()) printf("YES\n");
62         else printf("NO\n");
63     }
64 
65     return 0;
66 }

 1 //这种遍历所有路径的方法一般会超时，真的超时了，嘎嘎
 2 #include<iostream>
 3 #include<cstdio>
 4 #include<cstring>
 5 using namespace std;
 6 #define N 150
 7 int vis[N];
 8 int n ;
 9 struct Edge{
10     int to ;
11     int next;
12 }edge[N];
13 int head[N];
14 int Enct;
15 void init()
16 {
17     Enct = 0;
18     memset(head,-1,sizeof(head));
19     memset(vis,0,sizeof(vis));//标记0为未访问
20 }
21 void add(int from , int to )
22 {
23     edge[Enct].to = to;
24     edge[Enct].next = head[from];
25     head[from] = Enct++;
26 }
27 /*bool dfs(int i )
28 {
29     if(vis[i]) return false;
30     vis[i] = 1;
31     printf("vis[%d] = %d\n",i,vis[i]);
32     for(int j = head[i] ; j!=-1; j = edge[j].next)
33     {
34         Edge e = edge[j];
35         dfs(e.to);
36     }
37     return true;
38 }*/
39 bool tm = true;
40 bool dfs(int i )
41 {
42     vis[i]=1;
43     for(int j = head[i] ; j!=-1 ;j = edge[j].next)
44     {
45         Edge e = edge[j];
46         if(vis[e.to]==1) tm = false;
47         else
48         {
49             dfs(e.to);
50             vis[e.to]=0;//保证dfs走的是一条链，每次回溯的时候相当于走反向所以标记成未访问
51         }
52     }
53     return tm;
54 }
55 int main()
56 {
57     int m ;
58     while(~scanf("%d%d",&n,&m)&&(n!=0||m!=0))
59     {
60         init();
61         tm = true;
62         for(int i =0 ;i < m ;i++)
63         {
64             int a ,b;
65             scanf("%d%d",&a,&b);
66             add(a,b);
67         }
68         bool flag = true;
69         for(int i= 0; i < n ;i++)
70         {
71             if(flag == false) break;
72             if(vis[i]==0)
73                 flag = dfs(i);
74         }
75         if(flag) printf("YES\n");
76         else printf("NO\n");
77     }
78     return 0;
79 }