Submission #931835

#	Time	Username	Problem	Language	Result	Execution time	Memory
931835		vjudge1	Network (BOI15_net)	C++17	0 / 100	1 ms	776 KiB

net

#include <bits/stdc++.h>
#define file_io freopen("input.txt", "r", stdin);freopen("output.txt", "w", stdout);
#define fast_io ios::sync_with_stdio(false);cin.tie(0);
#define what(x) cerr << #x << " is " << x << '\n';
#define kill(x) {cout << x << endl; return 0;}
#define all(x) (x).begin(), (x).end()
#define pii pair<int, int>
#define pb push_back
#define ll long long
#define F first
#define S second
const ll inf = 1e18, mod = 1e9 + 7, delta = 1e9 + 9, SQ = 450, P = 6065621;
 
using namespace std;

const int N = 2005, LG = 40;
vector<pair<int, pii>> candies;
int par[N][LG], h[N];
vector<int> adj[N];
bool b[N];

void dfs (int v, int p = -1) {
  par[v][0] = p;
  if (p + 1) h[v] = h[p] + 1;
  for (int i = 1; i < LG; i++)
    if (par[v][i] + 1)
      par[v][i] = par[par[v][i - 1]][i - 1];
  for (auto u: adj[v])
    if (u - p) dfs(u, v);
}

int lca (int u, int v) {
  if (h[u] < h[v]) swap(u, v);
  for (int i = LG - 1; i >= 0; i--)
    if (par[u][i] + 1 && h[par[u][i]] >= h[v])
      u = par[u][i];
  if (u == v) return v;
  for (int i = LG - 1; i >= 0; i--)
    if (par[u][i] != par[v][i])
      v = par[v][i], u = par[u][i];
  return par[v][0];
}

bool cmp (pair<int, pii> x, pair<int, pii> y) {
  return x.F >= y.F;
}

int main () {
  fast_io;
  int n;
  cin >> n;
  memset(par, -1, sizeof par);
  for (int i = 1; i < n; i++) {
    int u, v;
    cin >> u >> v;
    adj[u].pb(v);
    adj[v].pb(u);
  }
  dfs(1);
  vector<int> l;
  for (int i = 1; i <= n; i++) {
    if (adj[i].size() == 1)
      l.pb(i);
  }
  for (int i = 0; i < l.size(); i++)
    for (int j = i + 1; j < l.size(); j++) {
      int u = l[i], v = l[j];
      candies.pb({h[v] + h[u] - 2 * h[lca(u, v)], {u, v}});
    } 
  sort(all(candies), cmp);
  cout << l.size() / 2 + (l.size() % 2 == 1) << '\n';
  set<int> s;
  for (auto u: l) s.insert(u);
  if (l.size() & 1) {
    for (auto e: candies) {
      if (b[e.S.F] || b[e.S.S]) continue;
      cout << e.S.F << ' ' << e.S.S << '\n';
      b[e.S.F] = b[e.S.S] = true;
      s.erase(e.S.F);
      s.erase(e.S.S);
    }
    int v = *s.begin();
    cout << v << ' ' << 1 << '\n';
  }
  else {
    for (auto e: candies) {
      if (b[e.S.F] || b[e.S.S]) continue;
      cout << e.S.F << ' ' << e.S.S << '\n';
      b[e.S.F] = b[e.S.S] = true;
    }
  }
  return 0;
}

Compilation message (stderr)

net.cpp: In function 'int main()':
net.cpp:65:21: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   65 |   for (int i = 0; i < l.size(); i++)
      |                   ~~^~~~~~~~~~
net.cpp:66:27: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   66 |     for (int j = i + 1; j < l.size(); j++) {
      |                         ~~^~~~~~~~~~

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