update

src/B4016/B4016.cpp

@@ -0,0 +1,63 @@
+#include <algorithm>
+#include <bitset>
+#include <cstdint>
+#include <iostream>
+#include <istream>
+#include <queue>
+#include <utility>
+#include <vector>
+
+using ll = int64_t;
+
+int main(){
+    std::iostream::sync_with_stdio(false), std::cin.tie(nullptr), std::cout.tie(nullptr);
+
+    const ll max_n = 1e5+5;
+    ll n;
+    std::cin>>n;
+    auto *edge = new std::vector<ll>[max_n];
+    ll a, b;
+    for(ll i{1}; i<n; i++){
+        std::cin>>a>>b;
+        edge[a].push_back(b);
+        edge[b].push_back(a);
+    }
+    std::queue<std::pair<ll, ll>> q;
+    q.emplace(0, a);
+    std::bitset<max_n> vis;
+    vis[a]=true;
+    ll max_a{}, max{};
+    while(!q.empty()){
+        auto top = q.front();
+        q.pop();
+        if(top.first > max){
+            max = top.first;
+            max_a = top.second;
+        }
+        for(auto &i:edge[top.second]){
+            if(vis[i]){
+                continue;
+            }
+            q.push({top.first + 1, i});
+            vis[i] = true;
+        }
+    }
+    q = std::queue<std::pair<ll, ll>>();
+    q.emplace(0, max_a);
+    vis.reset();
+    vis[max_a]=true;
+    ll ans{};
+    while(!q.empty()){
+        auto top = q.front();
+        q.pop();
+        ans = std::max(ans, top.first);
+        for(auto i: edge[top.second]){
+            if(vis[i]){
+                continue;
+            }
+            q.push({top.first+1, i});
+            vis[i] = true;
+        }
+    }
+    std::cout<<ans<<'\n';
+}