[AcWing 285] 没有上司的舞会

#include<iostream>
#include<cstring>

using namespace std;

const int N = 6010; 
int n;
int happy[N];
int h[N], e[N], ne[N], idx;
bool has_father[N];
int f[N][2];

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;
    return 0;
}