（连通图 缩点 强联通分支）Popular Cows -- poj --2186

#include<cstdio>
#include<cstring>
#include<cstdlib>
#include<iostream>
#include<queue>
#include<stack>
#include<algorithm>
using namespace std;

#define N 50005

struct node
{
    int v, next;
}a[N];

int Head[N], cnt;
int dfn[N], low[N], Time, bnt, belong[N];
int Stack[N], InStack[N], top;

void Init()
{
    cnt = Time = bnt = top = 0;
    memset(Head, -1, sizeof(Head));
    memset(dfn, 0, sizeof(dfn));
    memset(low, 0, sizeof(low));
    memset(Stack, 0, sizeof(Stack));
    memset(InStack, 0, sizeof(InStack));
}

void Add(int u, int v)
{
    a[cnt].v = v;
    a[cnt].next = Head[u];
    Head[u] = cnt++;
}

void Tarjar(int u)
{
    int v;
    low[u] = dfn[u] = ++Time;
    InStack[u] = 1;
    Stack[top++] = u;

    for(int j=Head[u]; j!=-1; j=a[j].next)
    {
        v = a[j].v;
        if(!dfn[v])
        {
            Tarjar(v);
            low[u] = min(low[u], low[v]);
        }
        else if(InStack[v])
            low[u] = min(low[u], dfn[v]);
    }

    if(dfn[u]==low[u])
    {
        bnt++;
        do
        {
            v = Stack[--top];
            InStack[v] = 0;
            belong[v] = bnt;
        }while(u!=v);
    }
}

int main()
{
    int n, m;
    while(scanf("%d%d", &n, &m)!=EOF)
    {
        int i, u, v;

        Init();
        for(i=1; i<=m; i++)
        {
            scanf("%d%d", &u, &v);
            Add(u, v);
        }

        for(i=1; i<=n; i++)
        {
            if(!dfn[i])
            Tarjar(i);
        }

        int Out[N]={0};
        for(int i=1; i<=n; i++)
        {
            for(int j=Head[i]; j!=-1; j=a[j].next)
            {
                u = belong[i], v = belong[a[j].v];
                if(u!=v)
                    Out[u]++;
            }
        }

        int flag=0, Index;
        for(i=1; i<=bnt; i++)
        {
            if(!Out[i])
            {
                flag++;
                Index = i;
            }
        }

        if(flag>1)
            printf("0\n");
        else
        {
            int ans = 0;
            for(i=1; i<=n; i++)
            {
                if(belong[i]==Index)
                    ans++;
            }
            printf("%d\n", ans);
        }
    }
    return 0;
}