90分 萌新求助

#include<bits/stdc++.h>
using namespace std;
const int N=10005,M=50005;
struct Edge{
	int v,nxt;
	}edge[M];
int cnt,tot,top,ans;	
int dfn[N],low[N],pre[N],sta[N],vis[N];
void add_for(int u,int v){
	edge[++cnt].nxt=pre[u];
	edge[cnt].v=v;
	pre[u]=cnt;
	return ;
	}
void tarjan(int x){
	dfn[x]=low[x]=++tot;
	sta[++top]=x;
	vis[top]=1;
	for(int i=pre[x];i!=-1;i=edge[i].nxt){
		if(!dfn[edge[i].v]){
			tarjan(edge[i].v);
			low[x]=min(low[x],low[edge[i].v]);
			}
		else if(vis[edge[i].v]){
			low[x]=min(low[x],low[edge[i].v]);
			}	
		}
	if(low[x]==dfn[x]){
		int k=0;
		do{
			k++;
			vis[sta[top]]=1;
			top--;
			}while(x!=sta[top+1]);
		if(k>1)ans++;
		}	
	return ;	
	}
int main(){
	std::ios::sync_with_stdio(false);
	memset(pre,-1,sizeof(pre));
	int n,m;
	cin>>n>>m;
	for(int i=1;i<=m;i++){
		int u,v;
		cin>>u>>v;
		add_for(u,v);
		}
	for(int i=1;i<=n;i++){
		if(!dfn[i])tarjan(i);
		}	
	cout<<ans;	
	return 0;
	}