HDU1269

 1 #include <cstdio>
 2 #include <cstring>
 3 #include <cstdlib>
 4 #include <cmath>
 5 #include <cctype>
 6 #include <time.h>
 7 #include <string>
 8 #include <set>
 9 #include <map>
10 #include <queue>
11 #include <vector>
12 #include <stack>
13 #include <algorithm>
14 #include <iostream>
15 using namespace std;
16 #define PI acos( -1.0 )
17 typedef long long ll;
18 typedef pair<int,int> P;
19 const double E = 1e-8;
20 
21 const int NO = 10000 +5;
22 vector <int> nod[NO];
23 int mark[NO];
24 int n, m;
25 
26 void dfs( int x )
27 {
28     mark[x] = 1;
29     for( int i = 0; i < nod[x].size(); ++i )
30         if( !mark[nod[x][i]] )
31             dfs( nod[x][i] );
32 }
33 
34 int main()
35 {
36     while( ~scanf( "%d%d", &n, &m ) && n+m )
37     {
38         int x, y;
39         while( m-- )
40         {
41             scanf( "%d%d", &x, &y );
42             nod[x].push_back( y );
43         }
44         bool flag = true;
45         for( int i = 1; i <= n; ++i )
46         {
47             memset( mark, 0, sizeof( mark ) );
48             dfs( i );
49             for( int j = 1; j <= n; ++j )
50                 if( !mark[j] )
51                 {
52                     flag = false;
53                     break;
54                 }
55             if( !flag )
56                 break;
57         }
58         if( flag )
59             puts( "Yes" );
60         else
61             puts( "No" );
62         for( int i = 1; i <= n; ++i )
63             nod[i].clear();
64     }
65     return 0;
66 }