POJ2186:Popular Cows

#include <iostream>
#include <string.h>
#include <stdio.h>
#include <algorithm>
#define N 100002
using namespace std;
int n,m,tt,time,cnt,e,a[N],b[N],sum[N];
struct node
{
    int x,y;
    int next;
} eg[1000001];
int head[N],low[N],dfn[N],f[N],instack[N],belong[N],s[N];
void init()
{
    memset(head,-1,sizeof(head));
    memset(instack,0,sizeof(instack));
    memset(belong,0,sizeof(belong));
    memset(sum,0,sizeof(sum));
    memset(f,0,sizeof(f));
    tt=0;
    cnt=0;
}
void add(int xx,int yy)
{
    eg[tt].x=xx;
    eg[tt].y=yy;
    eg[tt].next=head[xx];
    head[xx]=tt++;
}
void tarjan(int i)
{
    int w;
    dfn[i]=low[i]=++time;
    instack[i]=1;
    s[++e]=i;
    for(int j=head[i]; j!=-1; j=eg[j].next)
    {
        w=eg[j].y;
        if(!dfn[w])
        {
            tarjan(w);
            low[i]=min(low[i],low[w]);
        }
        else if(instack[w]==1)
        {
            low[i]=min(low[i],dfn[w]);
        }
    }
    if(dfn[i]==low[i])
    {
        cnt++;
        do
        {
            w=s[e--];
            instack[w]=0;
            belong[w]=cnt;
        }
        while(w!=i);
    }
}
void solve()
{
    time=e=0;
    memset(dfn,0,sizeof(dfn));
    for(int i=1; i<=n; i++)
    {
        if(!dfn[i])
        {
            tarjan(i);
        }
    }
}
int main()
{
    while(scanf("%d%d",&n,&m)!=EOF)
    {
        init();
        for(int i=1; i<=m; i++)
        {
            scanf("%d%d",&a[i],&b[i]);
            add(a[i],b[i]);
        }
        solve();
        for(int i=1; i<=m; i++)
        {
            if(belong[a[i]]!=belong[b[i]])
            {
                f[belong[a[i]]]=1;
            }
        }
        for(int i=1; i<=n; i++)
        {
            sum[belong[i]]++;
        }
        int V=0,count;
        for(int i=1; i<=cnt; i++)
        {
            if(!f[i])
            {
                V++;
                count=sum[i];
            }
        }
        if(V==1) printf("%d\n",count);
        else printf("0\n");
    }
    return 0;
}