思路：
树形dp。
实现：
1 #include <bits/stdc++.h> 2 using namespace std; 3 const int MAXN = 100005; 4 int n, a[MAXN], in[MAXN]; 5 vector<int> G[MAXN]; 6 double dp[MAXN][2]; 7 bool vis[MAXN]; 8 void dfs(int now) 9 { 10 vis[now] = true; 11 for (int i = 0; i < G[now].size(); i++) 12 { 13 int tmp = G[now][i]; 14 if (!vis[tmp]) dfs(tmp); 15 dp[now][0] += max(dp[tmp][0], dp[tmp][1] + a[tmp]); 16 dp[now][1] += max(dp[tmp][0], dp[tmp][1] + (double)a[tmp] / 2.0); 17 } 18 } 19 int main() 20 { 21 cin >> n; 22 int u, v; 23 for (int i = 1; i <= n; i++) cin >> a[i]; 24 for (int i = 0; i < n - 1; i++) { cin >> u >> v; in[v]++; G[u].push_back(v); } 25 int i = 1; 26 for (; i <= n; i++) if (!in[i]) break; 27 dfs(i); 28 printf("%.1f ", max(dp[i][0], dp[i][1] + a[i])); 29 return 0; 30 }