more code

codeforces/991/g.cc

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+#include <bits/stdc++.h>
+using namespace std;
+
+#define all(x) (x).begin(), (x).end()
+#define sz(x) static_cast<int>((x).size())
+#define FOR(i, x) for (int i = 0; i < (x).size(); ++i)
+
+template <typename T> T max() { return numeric_limits<T>::max(); }
+template <typename T> T min() { return numeric_limits<T>::min(); }
+
+#ifdef LOCAL
+#define dbg(x) cerr << #x << " = " << (x) << '\n'
+#else
+#define dbg(x)
+#endif
+
+void solve() {
+  int n;
+  cin >> n;
+
+  unordered_map<int, vector<int>> graph;
+
+  while (--n) {
+    int u, v;
+    cin >> u >> v;
+
+    graph[u].push_back(v);
+    graph[v].push_back(u);
+  }
+
+  auto ans = min<int>();
+
+  auto is_leaf = [&graph](int u) {
+    return (u != 1 && graph[u].size() == 1) || (u == 1) && graph[u].empty();
+  };
+
+  auto dfs = [&](auto &dfs, int u, int parent) -> int {
+    if (u == parent || is_leaf(u)) {
+      return 0;
+    }
+
+    int first = 0, second = 0;
+
+    for (auto v : graph[u]) {
+      if (v == parent)
+        continue;
+
+      auto through = dfs(dfs, v, u);
+
+      if (through > first) {
+        second = through;
+        swap(first, second);
+      } else if (through > second) {
+        second = through;
+      }
+    }
+
+    int remove_u_and_one_child = sz(graph[u]) - (u != 1) - (first != 0) + first;
+    int remove_through_u =
+        sz(graph[u]) - (u != 1) - (first != 0) - (second != 0) + first + second;
+    int no_remove_u = 1 + first;
+
+    ans = max(ans, max(remove_through_u, no_remove_u));
+
+    return remove_u_and_one_child;
+  };
+
+  std::cout << max(ans, dfs(dfs, 1, -1)) << '\n';
+}
+
+int main() {
+  cin.tie(nullptr)->sync_with_stdio(false);
+
+  int t;
+  cin >> t;
+
+  while (t--) {
+    solve();
+  }
+
+  return 0;
+}