ICPC Asia Shenyang 2019 Dudu's maze

#include <bits/stdc++.h>
#define ll long long
using namespace std;
const int maxn = 1e5+5;
vector<int> to[maxn];
int pos[maxn],fa[maxn];
bool vis[maxn];
bool book[maxn];
int n,m,k;
int cnt;
double ans,mx,num;
void init()
{
	for(int i=0;i<=n;i++){
		to[i].clear();
		vis[i] = false;
		book[i] = false;
		fa[i] = 0;
	}
}
void dfs1(int u){
	book[u] = 1;
	if(vis[u]){
		pos[++cnt] = u;
		return ;
	}
	ans++;
	for(int i=0;i<to[u].size();i++){
		int nx = to[u][i];
		if(book[nx])continue;
		dfs1(nx);
	}
}
void dfs2(int u,int pos){
	if(book[u] || vis[u]) return;
	fa[u] = pos; 
	num++;	
	for(int i=0;i<to[u].size();i++){
		int nx = to[u][i];
		if(fa[nx]==pos)continue;
		dfs2(nx,pos);
	}
}
int main()
{
	int _;
	scanf("%d",&_);
	while(_--){
		init(); 
		scanf("%d %d %d",&n,&m,&k);
		for(int i=1;i<=m;i++){
			int u,v;
			scanf("%d %d",&u,&v);
			to[u].push_back(v);
			to[v].push_back(u);
		}
		ans = 0;	cnt = 0;	mx = 0;
		for(int i=1;i<=k;i++){
			int tp;
			scanf("%d",&tp);
			vis[tp] = true;
		}	
		dfs1(1);
		int v=0;	
		for(int i=1;i<=cnt;i++){
			num =0;
			for(int j=0;j<to[pos[i]].size();j++){
				dfs2(to[pos[i]][j],++v);	
			}
			mx = max(mx,num/to[pos[i]].size());
		}
		printf("%.6lf\n",ans+mx);
	}
	return 0;
 }