水题，，直接贴代码。
//poj 3180//sep9#include <iostream>#include <stack>using namespace std;const int maxN=10024;const int maxM=50048;int sum[maxN];int head[maxN],dfn[maxN],low[maxN],ins[maxN],belong[maxN]; stack<int> s;struct Edge{int v,next;}edge[maxM];int n,m,e,t,cnt;void tarjan(int u){dfn[u]=low[u]=++t;s.push(u);ins[u]=1;for(int i=head[u];i!=-1;i=edge[i].next){int v=edge[i].v;if(dfn[v]==0){tarjan(v);low[u]=min(low[u],low[v]);}else if(ins[v]==1)low[u]=min(low[u],dfn[v]);}int k;if(low[u]==dfn[u]){++cnt;do{k=s.top();s.pop();ins[k]=0;belong[k]=cnt;}while(low[k]!=dfn[k]);}return ;}int main(){int n,m;scanf("%d%d",&n,&m); e=0;memset(head,-1,sizeof(head));memset(dfn,0,sizeof(dfn));memset(ins,0,sizeof(ins)); while(m–){int a,b;scanf("%d%d",&a,&b);edge[e].v=b;edge[e].next=head[a];head[a]=e++;}t=cnt=0;for(int i=0;i<n;++i)if(!dfn[i])tarjan(i);memset(sum,0,sizeof(sum));for(int i=0;i<n;++i)++sum[belong[i]];int ans=0;for(int i=1;i<=cnt;++i)if(sum[i]>1)++ans;printf("%d",ans);return 0;}
纵然伤心，也不要愁眉不展，因为你不知是谁会爱上你的笑容
