（树形DP入门）Anniversary party

There is going to be a party to celebrate the 80-th Anniversary of the Ural State University. The University has a hierarchical structure of employees. It means that the supervisor relation forms a tree rooted at the rector V. E. Tretyakov. In order to make the party funny for every one, the rector does not want both an employee and his or her immediate supervisor to be present. The personnel office has evaluated conviviality of each employee, so everyone has some number (rating) attached to him or her. Your task is to make a list of guests with the maximal possible sum of guests' conviviality ratings.

Input

Employees are numbered from 1 to N. A first line of input contains a number N. 1 <= N <= 6 000. Each of the subsequent N lines contains the conviviality rating of the corresponding employee. Conviviality rating is an integer number in a range from -128 to 127. After that go T lines that describe a supervisor relation tree. Each line of the tree specification has the form:
L K
It means that the K-th employee is an immediate supervisor of the L-th employee. Input is ended with the line

0 0

Output

Output should contain the maximal sum of guests' ratings.

Sample Input

7
1
1
1
1
1
1
1
1 3
2 3
6 4
7 4
4 5
3 5
0 0

Sample Output

5

题解：

状态转移方程：

dp[head][1] += dp[*it][0];

dp[head][0] += max(dp[*it][0],dp[*it][1]);（*it是子结点）

代码：

#include <iostream>
#include <cstring>
#include <cstdio>
#include <cmath>
#include <vector>

using namespace std;

const int MAXN = 6005;

vector<int> p[MAXN];//记录每个点的下属 
bool book[MAXN];//记录点是否有上司，用于找根结点 
int dp[MAXN][2];//0 表示该点不参加的最大ratings，1表示该点参加的。 

void treeDP(int head)
{
	if(p[head].empty())
	{
		return ;
	}

	vector<int>::iterator it;
	for(it=p[head].begin() ; it!=p[head].end() ; it++)
	{
		treeDP(*it);
		dp[head][1] += dp[*it][0];
		dp[head][0] += max(dp[*it][0],dp[*it][1]);
	}
}

int main()
{
	int N;
	while(scanf("%d",&N)!=EOF)
	{
		for(int i=1 ; i<MAXN ; i++)//清空p[] 
		{
			while(!p[i].empty())p[i].clear();
		}
		memset(book,false,sizeof(book));//赋值book[]为false 
		for(int i=1 ; i<=N ; i++)
		{
			scanf("%d",&dp[i][1]);
			dp[i][0] = 0;
		}
		int L,K;
		while(scanf("%d %d",&L,&K) && (L||K))
		{
			p[K].push_back(L);
			book[L] = true;
		}
		int head;
		for(int i=1 ; i<=N ; i++)//找头结点 
		{
			if(book[i] == false)
			{
				head = i;
				break;
			}
		}
		treeDP(head);
		printf("%d
",max(dp[head][0],dp[head][1]));
	}

	return 0;
}