python/TangDou/Topic/HuanGenDp/P2896.cpp

#include <bits/stdc++.h>
using namespace std;
const int N = 1e5 + 10, M = N << 1;
#define int long long
#define endl "\n"

// 链式前向星
int e[M], h[N], idx, w[M], ne[M];
void add(int a, int b, int c = 0) {
    e[idx] = b, ne[idx] = h[a], w[idx] = c, h[a] = idx++;
}

int f[N], sz[N], dis[N], c[N];

void dfs1(int u, int fa) {
    sz[u] = c[u];
    for (int i = h[u]; ~i; i = ne[i]) {
        int v = e[i];
        if (v == fa) continue;
        dis[v] = dis[u] + w[i]; // 每个点到根节点的距离
        dfs1(v, u);
        sz[u] += sz[v]; // 以u为根的子树奶牛数量
    }
}
void dfs2(int u, int fa) {
    for (int i = h[u]; ~i; i = ne[i]) {
        int v = e[i];
        if (v == fa) continue;
        f[v] = f[u] + (sz[1] - 2 * sz[v]) * w[i];
        dfs2(v, u);
    }
}
signed main() {
    // 初始化链式前向星
    memset(h, -1, sizeof h);

    int n;
    cin >> n;
    for (int i = 1; i <= n; i++) cin >> c[i];
    for (int i = 1; i < n; i++) {
        int a, b, c;
        cin >> a >> b >> c;
        add(a, b, c), add(b, a, c);
    }
    dfs1(1, -1);
    for (int i = 2; i <= n; i++) f[1] += dis[i] * c[i];

    dfs2(1, -1);
    int ans = 1e18;
    for (int i = 1; i <= n; i++) ans = min(ans, f[i]);
    cout << ans << endl;
}