强连通图 hdu 1269

***强连通图概念：从任意一点都可以到达任一点***

#include<cstdio>
#include<cstdlib>
#include<cmath>
#include<iostream>
#include<algorithm>
#include<cstring>
#include<vector>
using namespace std;
#define N 10005

int n, m;
int dfn[N], low[N];
int Stack[N], InStack[N];
int Time, top, cnt, bloks;

vector<vector<int> > G;

void Init()
{
    G.clear();
    G.resize(n+5);
    memset(dfn, 0, sizeof(dfn));
    memset(low, 0, sizeof(low));
    memset(Stack, 0, sizeof(Stack));
    memset(InStack, 0, sizeof(InStack));
    Time=top=cnt=bloks=0;
}

void Tarjan(int u)
{
    dfn[u]=low[u]=++Time;
    Stack[top++]=u;
    InStack[u]=1;
    int len=G[u].size(), v;

    for(int i=0; i<len; i++)
    {
        v=G[u][i];
        if(!dfn[v])
        {
            Tarjan(v);
            low[u]=min(low[u], low[v]);
        }
        else if(InStack[v])
        {
            low[u]=min(low[u], dfn[v]);
        }
    }
    if(dfn[u]==low[u])
    {
        do
        {
            cnt++;
            v=Stack[--top];
            InStack[v]=0;
        }while(u!=v);
        bloks++;
    }
}

int main()
{
    while(scanf("%d%d", &n, &m), n+m)
    {
        Init();
        int a, b;
        while(m--)
        {
            scanf("%d%d", &a, &b);
            G[a].push_back(b);
        }

        Tarjan(1);

        if(cnt==n&&bloks==1)
            printf("Yes
");
        else
            printf("No
");
    }
    return 0;
}