K - Anniversary party【樹形dp】

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 N – 1 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

思路：樹形dp入門，找到根節點後，從根節點開始dp，是其子節點就dfs再更新，一直到葉子節點。

#include <iostream>
#include <cstdio>
#include <algorithm>
using namespace std;
const int maxn=6005;

int n,x,y,dp[maxn][maxn],f[maxn],is[maxn],v[maxn];

void dfs(int root){
	v[root]=1;
	for(int i=1;i<=n;i++)
		if(v[i]==0&&f[i]==root)//如果i是root的兒子就進去 
		{
			dfs(i);
			dp[root][1]+=dp[i][0];                //  這兩行就是自下向上更新 。
			dp[root][0]+=max(dp[i][0],dp[i][1]); //1代表選，0代表不選。
		}
}

int main()
{
	int root=0;
	scanf("%d",&n);
	for(int i=1;i<=n;i++)
		scanf("%d",&dp[i][1]);
	while(scanf("%d%d",&x,&y))
	{
		if(x==0&&y==0)
			break;
		f[x]=y;
		is[x]++;
	}
	for(int i=1;i<=n;i++)//找根 
		if(is[i]==0){
			root=i;
			break;
		}
	dfs(root);
	printf("%d\n",max(dp[root][0],dp[root][1]));
}