题目描述

Description

Farmer John and his cows are planning to leave town for a long vacation, and so FJ wants to temporarily close down his farm to save money in the meantime.

The farm consists of barns connected with bidirectional paths between some pairs of barns (). To shut the farm down, FJ plans to close one barn at a time. When a barn closes, all paths adjacent to that barn also close, and can no longer be used.

FJ is interested in knowing at each point in time (initially, and after each closing) whether his farm is "fully connected" -- meaning that it is possible to travel from any open barn to any other open barn along an appropriate series of paths. Since FJ's farm is initially in somewhat in a state of disrepair, it may not even start out fully connected.

FJ和他的奶牛们正在计划离开小镇做一次长的旅行，同时FJ想临时地关掉他的农场以节省一些金钱。

这个农场一共有被用M条双向道路连接的N个谷仓(1<=N,M<=3000)。为了关闭整个农场，FJ 计划每一次关闭掉一个谷仓。当一个谷仓被关闭了，所有的连接到这个谷仓的道路都会被关闭，而且再也不能够被使用。

FJ现在正感兴趣于知道在每一个时间（这里的“时间”指在每一次关闭谷仓之后的时间）时他的农场是否是“全连通的”——也就是说从任意的一个开着的谷仓开始，能够到达另外的一个谷仓。注意自从某一个时间之后，可能整个农场都开始不会是“全连通的”。

Input

Output

4 3
1 2
2 3
3 4
3
4
1
2

YES
NO
YES
YES

#include <cstdio>
#include <cstring>
#include <cmath>
#include <queue>
#include <algorithm>
using namespace std;
int head[200100];
int to[400100];
int net[400100];
int a[200100];
int tot;
void add(int x,int y)
{
	net[++tot]=head[x];
	head[x]=tot;
	to[tot]=y;
}
int n,m;
bool open[200100];
int f[200100];
int find(int x)
{
	return f[x]==x?x:f[x]=find(f[x]);
}
int cnt;
int ans[200100];
int main()
{
	scanf("%d%d",&n,&m);
	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++)
		scanf("%d",&a[i]),f[i]=i;
	for(int i=n;i>=1;i--)
	{
		cnt++;
		open[a[i]]=true;
		for(int j=head[a[i]];j;j=net[j])
		{
			if(open[to[j]])
			{
				int dx=find(a[i]);
				int dy=find(to[j]);
				if(dx!=dy)
					f[dx]=dy,cnt--;
			}
		}
		if(cnt==1) ans[i]=1;
	}
	for(int i=1;i<=n;i++)
		if(ans[i]) printf("YES\n");
		else printf("NO\n");
    return 0;
}