tarjan代码的一个细节求问

#include<bits/stdc++.h>
using namespace std;
const int N=1e4+10,M=1e5+10;;
int h[N],ne[M],e[M],idx;
void add(int a,int b){
	e[++idx]=b,ne[idx]=h[a],h[a]=idx;
}
int n,m,cnt,Map[N];
vector<int> scc[N];
int dfn[N],low[N],timstamp;
stack<int> stk;
bool vis[N];
void tarjan(int u){
	dfn[u]=low[u]=++timstamp;
	stk.push(u); vis[u]=1;
	for(int j=h[u];j;j=ne[j]){
		int v=e[j];
		if(!dfn[v]){
			tarjan(v);
			low[u]=min(low[u],low[v]);
		}
		else if(vis[v]) low[u]=min(low[u],low[v]);
	}
	if(low[u]==dfn[u]){
		cnt++;
		while(!stk.empty()){
			int t=stk.top(); stk.pop(); vis[t]=0;
			scc[cnt].push_back(t); Map[t]=cnt;
			if(u==t) break;
		}
	}
}
bool flag[N];
int main(){
	ios::sync_with_stdio(0);
	cin.tie(0); cout.tie(0);
	cin>>n>>m;

	for(int i=1;i<=m;i++){
		int u,v; cin>>u>>v;
		add(u,v);
	}
	for(int i=1;i<=n;i++)
		if(!dfn[i]) tarjan(i);

	cout<<cnt<<endl;
	for(int i=1;i<=n;i++){
		if(flag[Map[i]]) continue;
		flag[Map[i]]=1;
		sort(scc[Map[i]].begin(),scc[Map[i]].end());
		for(int j=0;j<scc[Map[i]].size();j++)
			cout<<scc[Map[i]][j]<<' ';
		cout<<endl;
	}
	return 0;
}

else if(vis[v]) low[u]=min(low[u],low[v]);

else if(vis[v]) low[u]=min(low[u],dfn[v]);