这份边双为什么T了

#include<iostream>
#include<cstring>
#include<vector>
#include<algorithm>
#include<stack>
using namespace std;
struct edge {
	int u;int v;
};
stack<edge> s;
int pre[500000],bccno[500000],dfs_clock,vis[500000];
vector<int> g[500000],dfn[500000];
int bcc (int u,int fa) {
	int lowu=pre[u]=++dfs_clock;
	int child=0;
	for (int i=0;i<g[u].size();i++) {
		int v=g[u][i];
		if (!pre[v]) {
			child++;
			int lowv=bcc(v,u);
			lowu=min(lowv,lowu);
			if (lowv>pre[u]) {
				s.push((edge){u,v});
			}
		}
		else if (pre[v]<pre[u]&&v!=fa) lowu=min(lowu,pre[v]);
	}	
	return lowu;
}
int fb (int u,int dc) {
	bccno[u]=dc;
	vis[u]=1;
	for (int i=0;i<g[u].size();i++) {
		int v=g[u][i];
		if (v==-1) continue;
		if (vis[v]) continue;
		fb(v,dc);
	}
}
int find_bcc (int n) {
	vector<int> del[500000];
	while(!s.empty()) {
		edge e=s.top();
		s.pop();
		int u=e.u,v=e.v;
		//cout<<u<<' '<<v<<endl;
		del[u].push_back(v);
		del[v].push_back(u);
		for (int i=0;i<n;i++) {
			sort(g[i].begin(),g[i].end());
			sort(del[i].begin(),del[i].end());
		}
	}
	for (int i=1;i<=n;i++) {
		int p=0;
		for (int j=0;j<del[i].size();j++) {
			while (g[i][p]!=del[i][j]) {
				p++;
				if (p>=g[i].size()) break;
			}
			if (p<g[i].size()) g[i][p]=-1;
		}
	}
	dfs_clock=0;
	for (int i=1;i<=n;i++) if (!vis[i]) {
		//cout<<i<<' '<<dfs_clock<<endl;
		fb(i,++dfs_clock);
	}
}
int main() {
	int n,m;
	cin>>n>>m;
	for (int i=0;i<m;i++) {
		int f,t;
		cin>>f>>t;
		g[f].push_back(t);
		g[t].push_back(f);
	}
	for (int i=1;i<=n;i++) if (!pre[i]) bcc(i,-1);
	find_bcc(n);
	for (int i=1;i<=n;i++) dfn[bccno[i]].push_back(i);
	//for (int i=1;i<=n;i++) cout<<pre[i]<<' ';
	//cout<<endl;
	cout<<dfs_clock<<endl;
	for (int i=1;i<=dfs_clock;i++) {
		cout<<dfn[i].size()<<' ';
		for (int j=0;j<dfn[i].size();j++) cout<<dfn[i][j]<<' ';
		cout<<endl;
	}
	return 0;
}