Tarjan

int tp,s[N],in_stack[N];
int dfn[N],low[N],dfncnt;
int scc[N],sc,sz[N];
void tarjan(int u)
{
	dfn[u]=low[u]=++dfncnt;
	s[++tp]=u;
	in_stack[u]=1;
	for(int i=h[u];i;i=nxt[i])
	{
		int v=to[i];
		if(!dfn[v])
		{
			tarjan(v);
			low[u]=min(low[u],low[v]);
		}
		else if(in_stack[v])
		{
			low[u]=min(dfn[v],low[u]);
		}
	}
	if(dfn[u]==low[u])
	{
		sc++;
		while(s[tp]!=u)
		{
			scc[s[tp]]=sc;
			in_stack[s[tp]]=0;
			sz[sc]++;
			tp--;
		}
		scc[s[tp]]=sc;
		in_stack[s[tp]]=0;
		sz[sc]++;
		tp--;
	}
}

#include"bits/stdc++.h"
using namespace std;

const int N=1e6;
int n,m;
int f[N][2];

vector<int> tr[N<<1];
int cnt;
int h[N],to[N],nxt[N],tot;
void add(int x,int y)
{
	to[++tot]=y;
	nxt[tot]=h[x];
	h[x]=tot;
}

int low[N],dfn[N],dfncnt;
int s[N],tp;
void tarjan(int u)
{
	low[u]=dfn[u]=++dfncnt;
	s[++tp]=u;
	for(int i=h[u];i;i=nxt[i])
	{
		int v=to[i];
		if(!dfn[v])
		{
			tarjan(v);
			low[u]=min(low[u],low[v]);
			if(low[v]==dfn[u])
			{
				++cnt;
				for(int x=0;x!=v;--tp)
				{
					x=s[tp];
					tr[cnt].push_back(x);
					tr[x].push_back(cnt);
				}
				tr[cnt].push_back(u);
				tr[u].push_back(cnt);
			}
			else low[u]=min(low[u],dfn[v]);
			if(low[v]>dfn[u])
			{
				
			}
		}
	}
}

int main()
{
	scanf("%d%d",&n,&m);
	cnt=n;
	for(int i=1;i<=m;i++)
	{
		int x,y;
		scanf("%d%d",&x,&y);
		add(x,y);
		add(y,x);
	}
	for(int i=1;i<=n;i++)
	{
		if(!dfn[i]) 
		{
			tarjan(i),--tp;
		}
	}
	
	return 0;
}