HDU1269 迷宫城堡 判断是否为强连通图 图论基础题

/*
*State: HDU1269    46MS    1428K    1364 B
*题目大意：
*        给一个n个点，m条边的有向图，判断该图是否为强连通图
*解题思路：
*        用tarjan,但是要注意该图可能有连通分量。
*/

#include <iostream>
#include <vector>
#include <stdio.h>
#include <stack>
using namespace std;

const int MAX = 10005;
vector<int> vec[MAX];
stack<int> S;
int dfn[MAX], low[MAX], step, in[MAX];
int Flag;

void addEdge(int u, int v)
{
    vec[u].push_back(v);
}

void tarjan(int n)
{
    dfn[n] = low[n] = ++step;
    S.push(n);
    in[n]++;
    for(unsigned i = 0; i < vec[n].size(); i++)
    {
        int son = vec[n][i];
        if(dfn[son] == -1)
        {
            tarjan(son);
            low[n] = min(low[n], low[son]);
        }
        else if(in[son] != 0)
            low[n] = min(low[n], dfn[son]);
    }
    if(dfn[n] == low[n])
    {
        Flag++;
        int tmp;
        do
        {
            tmp = S.top();
            in[tmp]--;
            S.pop();
        }while(tmp != n && !S.empty());
    }
}

void init()
{
    Flag = 0;
    step = 0;
    while(!S.empty())
        S.pop();
    for(int i = 0; i < MAX; i++)
    {
        dfn[i] = low[i] = -1;
        vec[i].clear();
        in[i] = 0;
    }
}

int main(void)
{
#ifndef ONLINE_JUDGE
    freopen("in.txt", "r", stdin);
#endif
    
    int n, m;
    while(scanf("%d %d", &n, &m), n || m)
    {
        init();
        for(int i = 0; i < m; i++)
        {
            int u, v;
            scanf("%d %d", &u, &v);
            addEdge(u, v);
        }
        for(int i = 1; i <= n; i++)
        {
            if(dfn[i] == -1)
                tarjan(i);
        }
        if(Flag == 1)
            printf("Yes\n");
        else
            printf("No\n");
    }
    return 0;
}