Codeforces 743 D Chloe and pleasant prizes

题目地址：http://codeforces.com/contest/743/problem/D
题意：告诉你一个树每个节点的价值，让你求出不相交的两个子树的价值和最大。
思路：树形dp入门。用vector存树的信息，每次遍历的时候不走回头路，dp数组去存这个节点下最大的子树价值和。

#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#define LL long long 
#define N 200010
#define M 50010
#define inf 0x3f3f3f3f3f3f3f3f
using namespace std;
const LL mod = 1e9 + 7;
const double eps = 1e-9;
vector tree[N];
LL dp[N], val[N];
LL ans;
void dfs(int u, int pre) {
    for (int i = 0; i < tree[u].size(); i++) {
        int v = tree[u][i];
        if (v == pre) {
            continue;
        }
        dfs(v, u);
        val[u] += val[v];
        if (dp[u] != -inf) {
            ans = max(ans, dp[u] + dp[v]);
        }
        dp[u] = max(dp[u], dp[v]);
    }
    dp[u] = max(dp[u], val[u]);
}
int main() {
    cin.sync_with_stdio(false);
    LL n, u, v;
    while (cin >> n) {
        ans = -inf;
        for (int i = 1; i <= n; i++) {
            cin >> val[i];
            dp[i] = -inf;
            tree[i].clear();
        }
        for (int i = 1; i < n; i++) {
            cin >> u >> v;
            tree[u].push_back(v);
            tree[v].push_back(u);
        }
        dfs(1, -1);
        if (ans == -inf) {
            cout << "Impossible" << endl;
        }
        else {
            cout << ans << endl;
        }
    }
    return 0;
}