1073. 树的中心

https://www.acwing.com/problem/content/submission/code_detail/13212944/

AcWing/1073/1073.cpp

@@ -0,0 +1,74 @@
+#include <iostream>
+#include <vector>
+
+using std::cin;
+using std::cout;
+const char endl = '\n';
+
+const int N = 10005,
+          INF = 0x3f3f3f3f;
+
+int n, d1[N], d2[N], p[N], up[N];
+std::vector<std::pair<int, int>> g[N];
+
+int dfs1(int u, int fa) {
+    if (~fa && g[u].size() <= 1) return 0;
+
+    d1[u] = d2[u] = -INF;
+
+    for (auto e : g[u]) {
+        int v = e.first,
+            w = e.second;
+
+        if (v == fa) continue;
+
+        int d = dfs1(v, u) + w;
+        if (d >= d1[u]) {
+            d2[u] = d1[u];
+            d1[u] = d;
+            p[u] = v;
+        } else if (d > d2[u]) {
+            d2[u] = d;
+        }
+    }
+
+    return d1[u];
+}
+
+void dfs2(int u, int fa) {
+    for (auto e : g[u]) {
+        int v = e.first;
+        int w = e.second;
+
+        if (v == fa) continue;
+
+        up[v] = std::max(up[u], p[u] == v ? d2[u] : d1[u]) + w;
+
+        dfs2(v, u);
+    }
+}
+
+int main() {
+    std::ios::sync_with_stdio(false);
+
+    cin >> n;
+    for (int i = 1, u, v, w; i < n; i++) {
+        cin >> u >> v >> w;
+
+        g[u].push_back(std::make_pair(v, w));
+        g[v].push_back(std::make_pair(u, w));
+    }
+
+    dfs1(1, -1);
+    dfs2(1, -1);
+
+    int ans = INF;
+
+    for (int i = 1; i <= n; i++) {
+        ans = std::min(ans, std::max(d1[i], up[i]));
+    }
+
+    cout << ans << endl;
+
+    return 0;
+}