hihocoder1779 公路收费

思路：

枚举每个点做根即可。

实现：

 #include <bits/stdc++.h>
 using namespace std;
 typedef long long ll;
 const ll INF = 0x3f3f3f3f3f3f3f3f;
 ll a[], cnt[], buf[];
 int n, k;
 struct edge
 {
     int id, to; ll cost;
     edge(int id, int to, ll cost): id(id), to(to), cost(cost) {}
 };
 vector<edge> G[];
 void dfs(int u, int f)
 {
     for (int i = ; i < G[u].size(); i++)
     {
         int id = G[u][i].id, v = G[u][i].to; ll c = G[u][i].cost;
         if (v == f) continue;
         dfs(v, u);
         cnt[u] += cnt[v];
         buf[id] = cnt[v] * c;
     }
     cnt[u] += a[u];
 }
 bool cmp(ll a, ll b)
 {
     return a > b;
 }
 int main()
 {
     cin >> n >> k;
     for (int i = ; i <= n; i++) cin >> a[i];
     int x, y; ll w;
     for (int i = ; i < n - ; i++)
     {
         cin >> x >> y >> w;
         edge e(i, y, w);
         G[x].push_back(e);
         edge e1(i + n - , x, w);
         G[y].push_back(e1);
     }
     ll ans = INF;
     for (int i = ; i <= n; i++)
     {
         memset(buf, , sizeof buf);
         memset(cnt, , sizeof cnt);
         dfs(i, );
         sort(buf, buf +  * n - , cmp);
         ll tmp = ;
         for (int i = ; i <  * n - ; i++)
         {
             if (i < k) continue;
             tmp += buf[i];
         }
         ans = min(ans, tmp);
     }
     cout << ans << endl;
     return ;
 }