HDU 1520 Anniversary party

题意：每个节点有权值，子节点和父节点不能同时选，问最后能选的最大价值是多少

思路：树形DP入门，由于子节点和父节点不能同时选，那么决策其实就一目了然，对于第i个结点，选还是不选。dp[i][0]为不选取第i个结点时候的最大价值，dp[i][1]为选第i结点的最大价值，那么dp[i][0]+=max(dp[j][1],dp[j][0])，不选第i个结点的时候意味着你可能选了j结点，或者j结点也不选，dp[i][1]+=dp[j][0],j选了i就不能选

#include <cstdio>
#include <queue>
#include <cstring>
#include <iostream>
#include <cstdlib>
#include <algorithm>
#include <vector>
#include <map>
#include <string>
#include <set>
#include <ctime>
#include <cmath>
#include <cctype>
using namespace std;
#define maxn 6050
#define LL long long
int cas=1,T;
vector<int> e[maxn];
int f[maxn];
int a[maxn];
int dp[maxn][2];
void dfs(int u)
{
dp[u][1]=a[u];
for (int i = 0;i<e[u].size();i++)
{
int v = e[u][i];
dfs(v);
dp[u][0]+=max(dp[v][1],dp[v][0]);
dp[u][1]+=dp[v][0];
}
}
int main()
{
int n;
while (scanf("%d",&n)!=EOF)
{
for (int i = 1;i<=n;i++)
{
scanf("%d",&a[i]);
e[i].clear();
f[i]=-1;
dp[i][0]=dp[i][1]=0;
}
int x,y;
while (scanf("%d%d",&x,&y)&&x&&y)
{
f[x]=y;
e[y].push_back(x);
}
int temp = 1;
while (f[temp]!=-1)
temp=f[temp];
dfs(temp);
printf("%d\n",max(dp[temp][1],dp[temp][0]));
}
//freopen("in","r",stdin);
//scanf("%d",&T);
//printf("time=%.3lf",(double)clock()/CLOCKS_PER_SEC);
return 0;
}

Description

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