1 #include<stdio.h>
2 #include<stack>
3 #include<string.h>
4 #include<stdlib.h>
5 #include<algorithm>
6 using namespace std;
7 const int maxn = 10005;
8 const int maxm = 100005;
9 struct node{
10 int u,next;
11 }edge[ maxm ];
12 int head[ maxn ],cnt;
13 int vis[ maxn ],dfn[ maxn ],low[ maxn ],cnt2,id;
14 int n,m;
15
16 void init(){
17 cnt=0;
18 id=0;
19 cnt2=0;
20 memset( head,-1,sizeof( head ));
21 memset( vis,0,sizeof( vis ));
22 memset( dfn,-1,sizeof( dfn ));
23 memset( low,0,sizeof( low ));
24 }
25 void addedge( int a,int b ){
26 edge[ cnt ].u=b;
27 edge[ cnt ].next=head[ a ];
28 head[ a ]=cnt++;
29 }
30 stack<int>s;
31
32 void tarjan( int now ){
33 dfn[ now ]=low[ now ]=id++;
34 vis[ now ]=1;
35 s.push( now );
36 for( int i=head[ now ];i!=-1;i=edge[ i ].next ){
37 int next=edge[ i ].u;
38 if( dfn[ next ]==-1 ){
39 tarjan( next );
40 low[ now ]=min( low[ now ],low[ next ] );
41 }
42 else if( vis[ next ]==1 ){
43 low[ now ]=min( low[ now ],dfn[ next ] );
44 }
45 }
46 if( dfn[ now ]==low[ now ] ){
47 cnt2++;
48 while( 1 ){
49 int tmp=s.top();
50 s.pop();
51 vis[ tmp ]=0;
52 if( tmp==now ) break;
53 }
54 }
55 }
56
57 int main(){
58 while( scanf("%d%d",&n,&m)==2 &&(n+m) ){
59 init();
60 int a,b;
61 while( m-- ){
62 scanf("%d%d",&a,&b);
63 addedge( a,b );
64 }
65 for( int i=1;i<=n;i++ )
66 if( dfn[ i ]==-1 )
67 tarjan( i );
68 if( cnt2==1 ) printf("Yes\n");
69 else printf("No\n");
70 }
71 return 0;
72 }
View Code
