【USACO16OPEN】关闭农场Closing the Farm 并查集+邻接表

题目描述

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”。

输入输出格式

输入格式：
The first line of input contains and . The next lines each describe a

path in terms of the pair of barns it connects (barns are conveniently numbered

). The final lines give a permutation of

describing the order in which the barns will be closed.

输出格式：
The output consists of lines, each containing “YES” or “NO”. The first line

indicates whether the initial farm is fully connected, and line indicates

whether the farm is fully connected after the th closing.

输入输出样例

输入样例#1：
4 3
1 2
2 3
3 4
3
4
1
2
输出样例#1：
YES
NO
YES
YES

题解：如果判断每删除一个点的时候，图是否联通，也可以反着来思考。从最后开始循环，每当加入一个点的时候，能否保证已经加入的点是联通的，那么便可以用并查集来实现。每当加入一个点，将与该点联通的点加入并查集，看已经在图里的点是否和该并查集相连，便可得出答案。

代码：

#include
#include
#define N 200006
using namespace std;
struct node
{
    int from,to,next;
}e[N*2];
int n,m;
int e_num,head[N],a[N];
bool flag[N],ans[N];
int father[N];
int get()
{
    int x=0,f=1;
    char c;
    c=getchar();
    if (c==' ') return x*f;
    while (c<'0'||c>'9') {if (c=='-') f=-1;c=getchar();}
    while (c>='0'&&c<='9') {x=x*10+c-'0';c=getchar();}
    return x*f;
}
int find(int x)
{
    if (father[x]!=x) father[x]=find(father[x]);
    return father[x];
}
void mix(int x,int y)
{
    int xx,yy;
    xx=find(x);
    yy=find(y);
    father[yy]=xx;
}
void add(int from,int to)
{
    ++e_num;
    e[e_num].from=from;
    e[e_num].to=to;
    e[e_num].next=head[from];
    head[from]=e_num;
}
int main()
{
    int x,y,i,j,tmp;
    n=get();m=get();
    for (i=1;i<=m;i++)
    {
        x=get();y=get();
        add(x,y);
        add(y,x);
    }
    for (i=n;i>=1;i--)
    {
        a[i]=get();
        father[i]=i;
    }   
    int sum=1;
    for (i=1;i<=n;i++)
    {
        x=a[i];
        flag[x]=true;
        for (j=head[x];j;j=e[j].next)
        {
            if (find(e[j].to)!=find(x)&&flag[e[j].to])
            {
                mix(e[j].to,x);
                sum++;
            }
        }
        if (sum==i) ans[i]=1;
        else ans[i]=0;
    }
    for (i=n;i>=1;i--)
    {
        if (ans[i]==1) printf("YES\n");
        else printf("NO\n");
    }
}