Submission #940070

#	Time	Username	Problem	Language	Result	Execution time	Memory
940070		vjudge1	Network (BOI15_net)	C++17	100 / 100	551 ms	89476 KiB

net

/// ITNOG
#include <bits/stdc++.h>
//#include <ext/pb_ds/assoc_container.hpp>
//#include <ext/pb_ds/tree_policy.hpp>
//#pragma GCC optimize("O3,unroll-loops")
//#pragma GCC target("avx2,bmi,bmi2,lzcnt,popcnt")
#define F first
#define S second
#define pb push_back
#define ppb pop_back
#define fast_io ios::sync_with_stdio(false);cin.tie(NULL);
#define file_io freopen("input.txt","r",stdin);freopen("output.txt","w",stdout);
#define FOR(i,k,n) for(int i = k; i < n; ++ i)
#define debf cout<<"(0-0)\n";
#define all(x) x.begin(), x.end()
#define dec(x) cout << fixed << setprecision(x);
#define pf push_front
#define ppf pop_front
#define dash " ------- "
#define what(x) cerr << #x << " is " << x << endl;
#define eb emplace_back
//#define int short int
#define int long long
#define sz(s) (int) (s.size())
#define fl cout.flush()

using namespace std;
//using namespace __gnu_pbds;

typedef long long ll;
typedef pair <int, int> pii;
typedef pair <int, pii> pip;
typedef pair <pii, int> ppi;
typedef pair <ll, ll> pll;
typedef unsigned long long ull;
typedef long double ld;

template <class T> using max_heap = priority_queue <T, vector <T>, less <T> >;
template <class T> using min_heap = priority_queue <T, vector <T>, greater <T> >;
//template <class T> using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;

constexpr int MOD = 1e9 + 7, N = 5e5 + 8, M = 1e6, SQ = 600, INF = 1e15 + 8, LGN = 22, mod = 998244353, P = 131113;

mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());

int n, leaf, subleaf[N], r1;
vector <int> adj[N];
vector <int> v[N];
vector <pii> ans;
max_heap <pii> pq;

void dfs (int u, int par = -1){
  if (adj[u].size() == 1){
    ++ leaf;
    ++ subleaf[u];
  }
  for (int i : adj[u]){
    if (i != par){
      dfs (i, u);
      subleaf[u] += subleaf[i];
    }
  }
  return;
}

void dfs2 (int u, int par = -1){
  for (int i = 0; i < adj[u].size(); ++ i){
    int x = adj[u][i];
    if (x != par){
      if (subleaf[x] > leaf / 2){
        i = INF;
        r1 = x;
        dfs2 (x, u);
      }
    }
  }
  return;
}

void dfs3 (int u, int xx = -1, int par = -1){
  if (adj[u].size() == 1){
    v[xx].pb(u);
  }
  for (int i : adj[u]){
    if (i != par){
      if (u == r1){
        dfs3 (i, i, u);
      }
      else {
        dfs3 (i, xx, u);
      }
    }
  }
  return;
}

int32_t main(){
  fast_io;
  cin >> n;
  FOR (i, 0, n - 1){
    int aa, bb;
    cin >> aa >> bb;
    adj[--aa].pb(--bb);
    adj[bb].pb(aa);
    if (adj[aa].size() > 1){
      r1 = aa;
    }
  }
  dfs (r1);
  dfs2 (r1);
  cout << (leaf + 1) / 2 << '\n';
  dfs3 (r1);
  for (int i = 0; i < n; ++ i){
    if (v[i].size()){
      pq.push({v[i].size(), i});
    }
  }
  while (pq.size()){
    if (pq.size() == 1){
      break;
    }
    pii p1 = pq.top();
    pq.pop();
    pii p2 = pq.top();
    pq.pop();
    ans.pb({v[p1.S].back(), v[p2.S].back()});
    v[p1.S].ppb();
    v[p2.S].ppb();
    -- p1.F;
    -- p2.F;
    if (p1.F){
      pq.push(p1);
    }
    if (p2.F){
      pq.push(p2);
    }
  }
  if (pq.size() == 1){
    ans.pb({v[pq.top().S].back(), r1});
  }
  for (pii i : ans){
    cout << i.F + 1 << " " << i.S + 1 << '\n';
  }
  return 0;
}

// Yesterday is history
// Tomorrow is a mystery
// but today is a gift
// That is why it is called the present

Compilation message (stderr)

net.cpp: In function 'void dfs2(long long int, long long int)':
net.cpp:67:21: warning: comparison of integer expressions of different signedness: 'long long int' and 'std::vector<long long int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   67 |   for (int i = 0; i < adj[u].size(); ++ i){
      |                   ~~^~~~~~~~~~~~~~~

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