80pts 求助

#include<bits/stdc++.h>
#define int long long
using namespace std;
const int N=1e5+5;
const int M=1e6+5;
int n,m,k,l,cnt,tot=1,sum;
int a[N],b[N],dfn[N],low[N];
vector<int> edge[N];
vector<pair<int,int> > ans;
void tarjan(int u,int fa){
	dfn[u]=low[u]=++cnt;
	for(int v : edge[u]){
		if(!dfn[v]){
			tarjan(v,u);
			low[u]=min(low[u],low[v]);
			a[u]+=a[v],b[u]+=b[v];
			if(low[v]>dfn[u]){
				if(a[v]==l||b[v]==k||a[v]==0||b[v]==0)
					ans.push_back(make_pair(u,v));
			}
		}
		if(v!=fa)
			low[u]=min(low[u],dfn[v]);
	}
}
signed main(){
	cin>>n>>m>>k>>l;
	for(int i=1;i<=k;i++){
		int x;
		cin>>x,a[x]=1;
	}
	for(int i=1;i<=l;i++){
		int x;
		cin>>x,b[x]=1;
	}
	for(int i=1;i<=m;i++){
		int u,v;
		cin>>u>>v;
		edge[u].push_back(v);
		edge[v].push_back(u); 
	}
	tarjan(1,0);
	cout<<ans.size()<<'\n';
	for(pair<int,int> i : ans){
		cout<<i.first<<' '<<i.second<<'\n';
	}
	return 0;
}