P4408 [NOI2003] 逃学的小孩

https://www.luogu.com.cn/record/170994651

Luogu/P4408/P4408.cpp

@@ -0,0 +1,62 @@
+#include <iostream>
+#include <algorithm>
+#include <iterator>
+#include <vector>
+
+using std::cin;
+using std::cout;
+const char endl = '\n';
+
+const int N = 2e5 + 5;
+
+int n, m;
+long long ans;
+std::vector<std::pair<int, long long>> g[N];
+long long dep1[N]{-1}, dep2[N]{-1};
+
+void dfs(int u, int f, int w, long long *dep) {
+    dep[u] = dep[f] + w;
+
+    for (auto [v, w] : g[u]) {
+        if (v == f) continue;
+
+        dfs(v, u, w, dep);
+    }
+}
+
+int main() {
+    std::ios::sync_with_stdio(false);
+    cin.tie(nullptr);
+
+    cin >> n >> m;
+
+    for (int i = 1; i <= m; i++) {
+        int u, v;
+        long long w;
+
+        cin >> u >> v >> w;
+
+        g[u].emplace_back(v, w);
+        g[v].emplace_back(u, w);
+    }
+
+    dfs(1, 0, 1, dep1);
+
+    int a = std::max_element(dep1 + 1, dep1 + n + 1) - dep1;
+
+    dfs(a, 0, 1, dep1);
+
+    int b = std::max_element(dep1 + 1, dep1 + n + 1) - dep1;
+
+    dfs(b, 0, 1, dep2);
+
+    for (int i = 1; i <= n; i++) {
+        if (i == a || i == b) continue;
+
+        ans = std::max(ans, std::min(dep1[i], dep2[i]) + dep1[b]);
+    }
+
+    cout << ans << endl;
+
+    return 0;
+}