蒟蒻60ptsTLE+MLE+WA求助大佬

#include<bits/stdc++.h>
using namespace std;
const int N = 5e5+10,M = 1e6+10,md = 1e9+7;
int n,m,cnt,Siz,dfn[N],low[N],bri[M],f[N],siz[N],siz2[N];
long long int ans,dp[N][2],p[N];
vector<pair<int,int> > G[N];
vector<int> G2[N];
struct E{
	int x;
	int y;
}e[M];
int read(){
    char ch=getchar(); int res=0;
    while(!isdigit(ch)){ch=getchar();}
    while(isdigit(ch)){res=(res<<1)+(res<<3)+ch-'0';ch=getchar();}
    return res;
}
void Tarjan(int x,int fa){
	dfn[x]=low[x]=++cnt;
	for(int i=0;i<G[x].size();i++){
		int v=G[x][i].first,id=G[x][i].second;
		if(v==fa) continue;
		if(dfn[v]==0){
			Tarjan(v,x);
			low[x]=min(low[x],low[v]);
			if(dfn[x]<low[v]) bri[id]=bri[id^1]=1;
		}
		else{
			low[x]=min(low[x],dfn[v]);
		}
	}
}
void dfs(int x,int w){
	f[x]=w;
	siz[w]++;
	for(int i=0;i<G[x].size();i++){
		int v=G[x][i].first,id=G[x][i].second;
		if(f[v]==0&&bri[id]==0){
			dfs(v,w);
		}
	}
}
void dfs2(int x,int fa){
	dp[x][0]=1;
	dp[x][1]=p[siz[x]]-1;
	siz2[x]=1;
	for(int i=0;i<G2[x].size();i++){
		int v=G2[x][i];
		if(v==fa) continue;
		dfs2(v,x);
		siz2[x]+=siz2[v];
		long long int tmp=0,tmp2=0;
		tmp=(dp[x][0]%md*dp[v][0]%md*2)%md;
		tmp2=(dp[x][1]%md*dp[v][0]%md*2)%md;
		tmp2=(tmp2+dp[x][1]%md*dp[v][1]%md)%md;
		tmp2=(tmp2+dp[x][0]%md*dp[v][1]%md)%md;
		dp[x][0]=tmp;
		dp[x][1]=tmp2;
	}
	if(x!=1) ans=(ans+dp[x][1]%md*p[m-siz2[x]]%md)%md;
	else ans=(ans+dp[x][1]%md*p[m-siz2[x]+1]%md)%md;
}
signed main(){
	n=read();
	m=read();
	for(int i=1;i<=m;i++){
		int x,y;
		x=read();
		y=read();
		e[i].x=x;
		e[i].y=y;
		G[x].push_back(make_pair(y,i*2-1));
		G[y].push_back(make_pair(x,i*2));
	}
	for(int i=1;i<=n;i++){
		if(!dfn[i]){
			Tarjan(i,i);
		}
	}
	cnt=0;
	for(int i=1;i<=n;i++){
		if(!f[i]){
			dfs(i,++cnt);
		}
	}
	for(int i=1;i<=m;i++){
		if(f[e[i].x]!=f[e[i].y]){
			G2[f[e[i].x]].push_back(f[e[i].y]);
			G2[f[e[i].y]].push_back(f[e[i].x]);
		}
	}
	p[0]=1;
	for(int i=1;i<=M/2;i++){
		p[i]=(p[i-1]%md*2%md)%md;
	}
	dfs2(1,0);
	cout<<ans<<endl;
	return 0;
}