HDU1269-迷宫城堡

#include<iostream>
#include<cstring>
#include<cstdio>
#include<math.h>
#include<vector>
#include<algorithm>
#include<queue>
#include<set>
#include<string>
#include<string.h>
using namespace std;
#define maxn 10010

int n;
vector<int> g[maxn];

int Bcnt;
int Top;
int Index;
int low[maxn],dfn[maxn];
int belong[maxn],stack[maxn];
bool instack[maxn];

void init_tarjan()
{
    Bcnt=Top=Index=0;
    for(int i=1;i<=n;i++){
        low[i]=dfn[i]=0;
        g[i].clear();
    }
}

void tarjan(int u)
{
    stack[Top++]=u;
    instack[u]=1;
    low[u]=dfn[u]=++Index;
    for(int i=0;i<g[u].size();i++){
        int v=g[u][i];
        if(!dfn[v]){
            tarjan(v);
            low[u]=min(low[v],low[u]);
        }
        else if(instack[v]){
         low[u]=min(low[u],dfn[v]);
        }
    }
    if(low[u]==dfn[u]){
        ++Bcnt;
        int v;
        do{
            v=stack[--Top];
            instack[v]=0;
            belong[v]=Bcnt;
        }while(u!=v);
    }

}
int main()
{
    int m;
    while(cin>>n>>m,n+m){
        init_tarjan();
        while(m--){
            int x,y;
            scanf("%d%d",&x,&y);
            g[x].push_back(y);
        }

        for(int i=1;i<=n;i++)
            if(!dfn[i])
                tarjan(1);
      //  for(int i=1;i<=n;i++)
        //    printf("%d ",belong[i]);
        puts(Bcnt==1?"Yes":"No");
    }
    return 0;
}