树形DP

#include <iostream>
#include <algorithm>
#include <cstring>
using namespace std;
const int N = 6010;

int n, happy[N], h[N], ne[N], e[N], idx;
int f[N][2];
bool has_father[N];
void add(int a, int b)
{
    e[idx] = b, ne[idx] = h[a], h[a] = idx++;
}
void dfs(int u)
{
    f[u][1] = happy[u];
    for(int i = h[u]; i!=-1;i = ne[i])
    {
        int j = e[i];
        dfs(j);
        
        f[u][0] += max(f[j][0], f[j][1]);
        f[u][1] += f[j][0];
    }
}
int main()
{
    cin>>n;
    for(int i = 1; i <= n; i++) cin>>happy[i];
    
    memset(h, -1, sizeof h);
    
    for(int i = 0; i < n - 1; i++)
    {
        int a, b;
        cin>>a>>b;
        has_father[a] = true;
        add(b, a);
    }
    
    int root = 1;
    while(has_father[root]) root++;
    
    dfs(root);
    
    cout<<max(f[root][0], f[root][1])<<endl;
}