[HAOI2006]受欢迎的牛 Tarjan

#include <bits/stdc++.h>
using namespace std;
const int N = 20000;
const int M = 100000;
struct E{
	int nxt,v;
};
E edge[M];
int a[M],b[M];
int head[M],cnt,num,n,m,out[N];
int low[N],dfn[N],book[N],belong[N],sum[N];
stack<int> s;
void tarjan(int u){
	dfn[u] = low[u] = ++num;
	s.push(u); book[u]=1;
	for(int v,i=head[u];i;i=edge[i].nxt){
		v = edge[i].v;
		if(!dfn[v]){
			tarjan(v);
			low[u] = min(low[u],low[v]);
		}else if(book[v])low[u] = min(low[u],dfn[v]);
	}
	if(dfn[u]==low[u]){
		int v;
		++cnt;
		do{
		 v = s.top();
		 s.pop();
		 book[v] = 0;
		 belong[v] = cnt;
		 sum[cnt]++;
		}while(u!=v);
	}
}
int main()
{
	scanf("%d %d",&n,&m);
	for(int i=1;i<=m;i++){
		int u,v;
		scanf("%d %d",&a[i],&b[i]);
		u = a[i]; v = b[i];
		edge[i].v = v; edge[i].nxt = head[u]; head[u] = i;
	} 

	for(int i=1;i<=n;i++)
		if(!dfn[i]){
			tarjan(i);
		}	
	for(int i=1;i<=m;i++){
		if(belong[a[i]]!=belong[b[i]]){
			out[belong[a[i]]]++;
		}
	}
	int flag=0,id;
	for(int i=1;i<=cnt;i++){
		if(!out[i]){
			id = i;flag++;
			if(flag>=2){
				printf("0\n");return 0;
			}
		}
	}
	printf("%d\n",sum[id]);
	return 0;
}