luogu P2279 [HNOI2003]消防局的设立

$\text{I've known that S.H.E's just released }$beautiful world$\text{this morning in radio. }$$\text{But it only has live beta(That's too bad).}$

$\text{Description}$

$\text{Given interger }N\text{ and a tree with }N\text{ points. To give a definition of a watered point,}$
$\text{which has a special point to it in distance 2 or shorter.}$
$\text{Please place special points as less as possible, so that each point of them ALL got watered.}$

$\text{Solution 2279}$

$\text{Consider using Greedy Algorithm.}$
$\text{First of all, we push all the points in order by their deeps.}$
$\text{For each point, we just pull out the deepest unwatered point, and set its grandfather}$
$\text{to a special point. That's the optimal solution.}$


$\text{Don't forget to double the memory of the array of edges!}$

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<algorithm>

#define reg register

struct node{
	int x,y,next;
}e[2010];
int len=0;
int first[1010];
int h[1010];
int fa[1010];
int n;
int sd;
int ans=0;

void ins(int x,int y){
	e[++len].x=x;e[len].y=y;
	e[len].next=first[x];first[x]=len;
}
void dfs(int f,int x,int hi){
	if(h[x]) return;
	h[x]=hi;fa[x]=f;
	for(reg int i=first[x];i;i=e[i].next)
		dfs(x,e[i].y,hi+1);
}
struct nod{
	int id,h;
	friend bool operator<(const nod a,const nod b){
		return a.h>b.h;
	}
}p[1010];
int ok[1010];
void set(int x){
	for(reg int i=first[x];i;i=e[i].next){
		int y=e[i].y;
		ok[y]=1;
		for(reg int j=first[y];j;j=e[j].next){
			int z=e[j].y;
			ok[z]=1;
		}
	}
	ok[fa[x]]=1;ok[fa[fa[x]]]=1;
	for(reg int i=first[fa[x]];i;i=e[i].next){
		int y=e[i].y;
		ok[y]=1;
	}
}
void work(){
	for(reg int i=1;i<=n;++i)
		p[i]=(nod){i,h[i]};
	std::sort(p+1,p+n+1);
	for(reg int i=1;i<=n;++i){
		if(ok[p[i].id]) continue;
		++ans;
		set(fa[fa[p[i].id]]);
	}
}
int main(){
	memset(first,0,sizeof(first));
	scanf("%d",&n);
	for(reg int i=1;i<n;++i){
		scanf("%d",&sd);
		ins(i+1,sd);ins(sd,i+1);
	}
	memset(h,0,sizeof(h));
	dfs(1,1,1);
	work();
	printf("%d",ans);
}