HDU1269 (tarjan)

#include<stdio.h>
#include<stack>
#include<string.h>
#include<stdlib.h>
#include<algorithm>
using namespace std;
const int maxn = 10005;
const int maxm = 100005;
struct node{
    int u,next;
}edge[ maxm ];
int head[ maxn ],cnt;
int vis[ maxn ],dfn[ maxn ],low[ maxn ],cnt2,id;
int n,m;

void init(){
    cnt=0;
    id=0;
    cnt2=0;
    memset( head,-1,sizeof( head ));
    memset( vis,0,sizeof( vis ));
    memset( dfn,-1,sizeof( dfn ));
    memset( low,0,sizeof( low ));
}
void addedge( int a,int b ){
    edge[ cnt ].u=b;
    edge[ cnt ].next=head[ a ];
    head[ a ]=cnt++;
}
stack<int>s;

void tarjan( int now ){
    dfn[ now ]=low[ now ]=id++;
    vis[ now ]=1;
    s.push( now );
    for( int i=head[ now ];i!=-1;i=edge[ i ].next ){
        int next=edge[ i ].u;
        if( dfn[ next ]==-1 ){
            tarjan( next );
            low[ now ]=min( low[ now ],low[ next ] );
        }
        else if( vis[ next ]==1 ){
            low[ now ]=min( low[ now ],dfn[ next ] );
        }
    }
    if( dfn[ now ]==low[ now ] ){
        cnt2++;
        while( 1 ){
            int tmp=s.top();
            s.pop();
            vis[ tmp ]=0;
            if( tmp==now ) break;
        }
    }
}

int main(){
    while( scanf("%d%d",&n,&m)==2 &&(n+m) ){
        init();
        int a,b;
        while( m-- ){
            scanf("%d%d",&a,&b);
            addedge( a,b );
        }
        for( int i=1;i<=n;i++ )
            if( dfn[ i ]==-1 )
                tarjan( i );
        if( cnt2==1 ) printf("Yes\n");
        else printf("No\n");
    }
    return 0;
}