hdu 1269(tarjan)

#include <iostream>
#include <cstdio>
#include <cstring>
#include <cstdio>
#include <algorithm>
#include <vector>
#include <queue>
#include <cmath>
#include <stack>
#include <set>
using namespace std;
#define hh printf("--------------\n");
const int maxn=10000+100;
const int maxm=100000+100;
int low[maxn],dfn[maxn];
int ncnt,ccnt;
int noc[maxn];
int hade[maxn];
bool vis[maxn],insk[maxn];
stack<int> sk;
struct note
{
    int to;
    int next;
}aa[maxm];
int n,m,k;
void addage(int fr,int to)
{
    aa[k].to=to;
    aa[k].next=hade[fr];
    hade[fr]=k++;
}
void tarjan(int u)
{
    ncnt++;
    low[u]=ncnt;
    dfn[u]=ncnt;
    sk.push(u);
    vis[u]=1;
    insk[u]=1;
     for(int i=hade[u];i!=-1;i=aa[i].next)
     {
         int v=aa[i].to;
         if(!vis[v])
         {
             tarjan(v);
             low[u]=min(low[u],low[v]);
         }
         else if(insk[v])
         {
             low[u]=min(low[u],dfn[v]);
         }
     }
     if(low[u]==dfn[u])
     {
         int v;
         do
         {
             v=sk.top();
             sk.pop();
             insk[v]=1;
             noc[v]=ccnt;
         }
         while(v!=u);
            ccnt++;
     }
}
void init()
{
    k=0;
    memset(hade,-1,sizeof(hade));
    memset(vis,0,sizeof(vis));
    memset(insk,0,sizeof(insk));
    while(sk.size()) sk.pop();
    ncnt=0;
    ccnt=0;
}
int main()
{
      while(~scanf("%d%d",&n,&m)&&(m+n))
      {
            init();
            int a,b;
         for(int i=1;i<=m;i++)
         {
             scanf("%d%d",&a,&b);
             addage(a,b);
         }
           for(int i=1;i<=n;i++)
            if(!vis[i]) tarjan(i);
          if(ccnt==1) printf("Yes\n");
        else printf("No\n");
      }
    return 0;
}