连通图模板（HDU 1269）

#include <stdio.h>
#include <algorithm>
#include <string.h>
#include <stack>
#include <vector>
using namespace std;
#define N 10100
int Instack[N], dfn[N], low[N], block, cnt, Time, Top, my_Stack[N];
vector<vector<int> >G;
int n, m;
void Tra(int u)
{
    dfn[u] = low[u] = ++Time;
    Instack[u] = 1;
    my_Stack[Top++] = u;
    int len = G[u].size(), v;
    for(int i=0; i<len; i++)
    {
        v = G[u][i];
        if(!dfn[v])
        {
            Tra(v);
            low[u] = min(low[u], low[v]);
        }
        else if(Instack[v])
            low[u] = min(low[u], dfn[v]);
    }
    if(low[u]==dfn[u])
    {
        block++;
        do
        {
            cnt++;
            v = my_Stack[--Top];
            Instack[v] = 0;
        }while(u!=v);
    }
}
void Init()
{
    memset(dfn, 0, sizeof(dfn));
    memset(Instack, 0, sizeof(Instack));
    memset(low, 0, sizeof(low));
    memset(my_Stack, 0, sizeof(my_Stack));
    G.clear();
    G.resize(n+1);
    block = cnt = Time = Top = 0;
}
int main()
{
    while(scanf("%d %d", &n, &m), m+n)
    {
        int a, b;
        Init();
        for(int i=1; i<=m; i++)
        {
            scanf("%d %d", &a, &b);
            G[a].push_back(b);
        }
        Tra(1);
        if(block==1 && cnt==n)printf("Yes\n");
        else printf("No\n");
    }
    return 0;
}