P2986 [USACO10MAR] Great Cow Gathering G

https://www.luogu.com.cn/problem/P2986

Luogu/P2986/P2986.cpp

@@ -0,0 +1,81 @@
+#include <iostream>
+#include <vector>
+
+using std::cin;
+using std::cout;
+const char endl = '\n';
+
+const int N = 1e5 + 5;
+
+int n, sum, root, c[N], siz[N], max_siz[N], dep[N];
+long long ans;
+std::vector<std::pair<int, int>> g[N];
+
+void dfs1(int u, int f) {
+    siz[u] = c[u];
+
+    for (auto e : g[u]) {
+        int v = e.first,
+            w = e.second;
+
+        if (v == f) continue;
+
+        dfs1(v, u);
+
+        siz[u] += siz[v];
+        max_siz[u] = std::max(max_siz[u], siz[v]);
+    }
+
+    max_siz[u] = std::max(max_siz[u], sum - siz[u]);
+}
+
+void dfs2(int u, int f) {
+    for (auto e : g[u]) {
+        int v = e.first,
+            w = e.second;
+
+        if (v == f) continue;
+
+        dep[v] = dep[u] + w;
+        dfs2(v, u);
+    }
+}
+
+int main() {
+    std::ios::sync_with_stdio(false);
+    cin.tie(nullptr);
+
+    cin >> n;
+
+    for (int i = 1; i <= n; i++) {
+        cin >> c[i];
+
+        sum += c[i];
+    }
+
+    for (int i = 1, u, v, w; i <= n; i++) {
+        cin >> u >> v >> w;
+
+        g[u].emplace_back(v, w);
+        g[v].emplace_back(u, w);
+    }
+
+    dfs1(1, 0);
+
+    for (int i = 1; i <= n; i++) {
+        if (!root || max_siz[i] < max_siz[root]) {
+            root = i;
+        }
+    }
+
+    dep[0] = -1;
+    dfs2(root, 0);
+
+    for (int i = 1; i <= n; i++) {
+        ans += static_cast<long long>(c[i]) * dep[i];
+    }
+
+    cout << ans << endl;
+
+    return 0;
+}