题目地址：http://poj.org/problem?id=3180
求强连通分量的点数>=2的个数
#include<iostream>
#include<cstdio>
#include<vector>
#include<stack>
#include<cstring>
#include<algorithm>
using namespace std;
const int maxn=10000+10;
bool vis[maxn];
int ans,index;
vector<int> dfn(maxn),low(maxn),st;
vector<vector<int> > G(maxn);
void Tarjan(int u)
{dfn[u]=low[u]=++index;vis[u]=true;st.push_back(u);for(int i=0;i<G[u].size();i++){int v=G[u][i];if(!vis[v]){Tarjan(v);low[u]=min(low[u],low[v]);}else if(find(st.begin(),st.end(),v)!=st.end()) //in stacklow[u]=min(low[u],dfn[v]);}if(dfn[u]==low[u]){int v,ncolor=0;do{v=st.back(); st.pop_back();ncolor++;}while(v!=u);if(ncolor>=2) ans++;}
}
int main()
{int n,m;int u,v;cin>>n>>m;while(m--){scanf("%d%d",&u,&v);G[u].push_back(v);}for(int i=1;i<=n;i++)if(!vis[i]) Tarjan(i);cout<<ans;return 0;
}