1 #include<cstdio>
 2 #include<cstring>
 3 #include<iostream>
 4 #include<cstring>
 5 #include<vector>
 6 #include<stack>
 7 #include<algorithm>
 8 using namespace std;
 9  
10 #define N 10003
11 int dfn[N],low[N],ins[N],Time,num;//
 
ins是否在棧裏
12 vector<int>gra[N];
13 stack<int>sta;
14  
15 void Tarjan(int s)
16 {
17     dfn[s] = low[s] = ++Time;
18     sta.push(s);
19     ins[s] = 1;
20     for(int i=0;i<gra[s].size();i++)
21     {
22         int k = gra[s][i];
23         if(dfn[k] == 0){
24             Tarjan(k);
25
 
             low[s] = min(low[s] ,low[k]);
26         }
27         if(dfn[k] != 0 && ins[k] == 1){
28             low[s] = min(low[s] ,dfn[k]);//low[s] = min(low[s] ,low[k]);好像也是對的
29         }
30     }
31     if(dfn[s] == low[s])
32     {
33         num++;
34         while(!sta.empty())
35         {
 
36             int temp = sta.top();
37             sta.pop();
38             ins[temp] = 0;
39             if(temp == s) break;
40         }
41     }
42 }
43  
44 int main()
45 {
46     int n,m;
47     while(scanf("%d%d",&n,&m)&&(n + m))
48     {
49         memset(dfn,0,sizeof(dfn));
50         memset(ins,0,sizeof(ins));
51         memset(low,0,sizeof(low));
52         while(!sta.empty()) sta.pop();
53         for(int i=1;i<=n;i++) gra[i].clear();
54         int a,b;
55         while(m--)
56         {
57             scanf("%d%d",&a,&b);
58             gra[a].push_back(b);
59         }
60         num = Time = 0;
61         for(int i=1;i<=n;i++)
62         {
63             if(dfn[i]==0) Tarjan(i);
64         }
65         if(num > 1) printf("No\n");
66         else printf("Yes\n");
67     }
68 }

HDU1269迷宮城堡