BZOJ 1596--电话网络(树形DP)

#include<cstdio>
#include<cstring>
#include<cmath>
#include<algorithm>
#include<iostream>
#define N 10010
#define E 20020
using namespace std;
int n,cnt;
int head[N],dp[N][3];
struct edge{
    int r,next;
}e[E];
void insert(int u,int v){
    e[++cnt].r=v;
    e[cnt].next=head[u];
    head[u]=cnt;
}
void tree_DP(int x,int F){
    int sum=0;
    dp[x][1]=1;dp[x][0]=N;
    for(int i=head[x];i>0;i=e[i].next)
        if(e[i].r!=F){
            tree_DP(e[i].r,x);
            dp[x][1]+=min(dp[e[i].r][0],min(dp[e[i].r][1],dp[e[i].r][2]));
            dp[x][2]+=dp[e[i].r][0];
            sum+=min(dp[e[i].r][0],dp[e[i].r][1]);
        }
    for(int i=head[x];i>0;i=e[i].next)
        if(e[i].r!=F)
            dp[x][0]=min(dp[x][0],dp[e[i].r][1]+sum-min(dp[e[i].r][0],dp[e[i].r][1]));
}
int main(){
    int x,y,ans;
    cnt=0;
    scanf("%d",&n);
    for(int i=1;i<n;i++){
        scanf("%d%d",&x,&y);
        insert(x,y);
        insert(y,x);
    }
    tree_DP(1,0);
    ans=min(dp[1][0],dp[1][1]);
    printf("%d\n",ans);
    return 0;
}