Submission #855239

#	Time	Username	Problem	Language	Result	Execution time	Memory
855239		NeroZein	Network (BOI15_net)	C++17	0 / 100	2 ms	13144 KiB

net

#include "bits/stdc++.h"
using namespace std;

#ifdef Nero
#include "Deb.h"
#else
#define deb(...)
#endif

const int N = 5e5 + 5;

int dep[N]; 
vector<int> g[N]; 

int dfs(int v, int p) {
  int ret = v; 
  for (int u : g[v]) {
    if (u != p) {
      dep[u] = dep[v] + 1;
      int t = dfs(u, v); 
      if (dep[t] > dep[ret]) {
        ret = t; 
      }
    }
  }
  return ret; 
}

int main(){
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  int n;
  cin >> n;
  for(int i = 0; i < n - 1; ++i) {
    int u, v;
    cin >> u >> v;
    g[u].push_back(v);
    g[v].push_back(u);
  }
  int a = dfs(1, 1); 
  dep[a] = 0;
  int b = dfs(a, a);
  assert(g[a].size() == 1 && g[b].size() == 1); 
  vector<pair<int, int>> ans = {{a, b}}; 
  int t = -1;
  for (int i = 1; i <= n; ++i) {
    if ((int) g[i].size() > 1 || i == a || i == b) {
      continue;
    }
    if (t == -1) {
      t = i;
    } else {
      ans.emplace_back(t, i);
      t = -1;
    }
  }
  if (t != -1) {
    ans.emplace_back(1, t);
  }
  cout << ans.size() << '\n'; 
  for (auto [u, v] : ans) {
    cout << u << ' ' << v << '\n';
  }
  return 0;
}

• Subtask 1: 0 / 18
• Subtask 2: 0 / 45
• Subtask 3: 0 / 37