hdu 1269 迷宮城堡&nbsp…

//hdu 1269 迷宮城堡 tarjan算法求極大連通子圖
#include
#include
#include
using namespace std;
#define max 11000
vector V[max];
int n,m,top,index,ans;
int instack[max],stack[max],visit[max];
int dfn[max],low[max];
void init()
{
   top=ans=0;
   index=1;
   for(int i=0;i
      V[i].clear();
      visit[i]=0;
      instack[i];
   }
}
void tarjan(int u)
{
   int i,j,v;
   dfn[u]=low[u]=index++;
   stack[top++]=u;
   visit[u]=1;
   instack[u]=1;
   for(i=0;i
      v=V[u][i];
      if(!visit[v]){
          tarjan(v);
          low[u]=min(low[u],low[v]);
      }
      if(instack[v])
         low[u]=min(low[u],dfn[v]);
   }
   if(dfn[u]==low[u]){
      do{
            j=stack[top-1];
            instack[j]=0;
            top--;
      }while(j!=u);
      ans++;
   }
   if(ans>1)return;
}                 
int main()
{
    int i,j,x,y;
    while(scanf("%d%d",&n,&m),n+m){
       init();
       for(i=1;i<=m;i++){
         scanf("%d%d",&x,&y);
         V[x].push_back(y);
       }
       for(i=1;i<=n;i++){
         if(!visit[i])
            tarjan(i);
         if(ans>1)break;
       }
       if(ans==1||n==1)
         printf("Yes\n");