Submission #855242

#	Time	Username	Problem	Language	Result	Execution time	Memory
855242		NeroZein	Network (BOI15_net)	C++17	18 / 100	7 ms	27228 KiB

net

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

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

const int N = 5e5 + 5;

int a, b;
int pr[N]; 
int dep[N]; 
vector<int> g[N]; 
vector<int> vec[N];
bool from_diameter[N];
vector<pair<int,int>> ans;

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

vector<int> merge(vector<int>& x, vector<int>& y) {
  if (x.size() < y.size()) swap(x, y);
  for (int v : y) x.push_back(v);
  return x;
}

vector<int> extract(vector<int>& x, vector<int>& y) {
  if (x.size() < y.size()) swap(x, y);
  for (int i = 0; i < (int) y.size(); ++i) {
    ans.emplace_back(x.back(), y.back()); 
    y.pop_back();
    x.pop_back(); 
  }
  return x; 
}

void dfs2(int v, int p) {
  if ((int) g[v].size() == 1 && v != a && v != b) {
    vec[v].push_back(v); 
  }
  for (int u : g[v]) {
    if (u == p) {
      continue;
    }
    dfs2(u, v);
    if (!from_diameter[v]) {
      vec[v] = merge(vec[v], vec[u]); 
    } else {
      vec[v] = extract(vec[v], vec[u]); 
    }
  }
}

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);
  }
  a = dfs(1, 1); 
  dep[a] = 0;
  b = dfs(a, a);
  int x = b;
  while (true) {
    from_diameter[x] = true;
    if (x == a) {
      break;
    }
    x = pr[x];
  }
  dfs2(a, a); 
  ans.emplace_back(a, b);
  for (int v : vec[a]) {
    ans.emplace_back(a, v); 
  }
  cout << (int) ans.size() << '\n';
  for (auto [u, v] : ans) {
    cout << u << ' ' << v << '\n'; 
  }
  return 0;
}

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