Submission #1155710

#	Time	Username	Problem	Language	Result	Execution time	Memory
1155710		fryingduc	Network (BOI15_net)	C++20	0 / 100	8 ms	12100 KiB

net

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

#ifdef duc_debug
#include "bits/debug.h"
#else
#define debug(...)
#endif

const int maxn = 5e5 + 5;
int n, h[maxn], tin[maxn], timer;
vector<int> g[maxn];
vector<int> leaves;

void pre_dfs(int u, int prev) {
  if ((int)g[u].size() == 1) {
    leaves.push_back(u);
  }
  tin[u] = ++timer;
  for (auto v:g[u]) {
    if (v == prev) continue;
    h[v] = h[u] + 1;
    pre_dfs(v, u);
  }
}

void solve() {
  cin >> n;
  int root = 0;
  for (int i = 1; i < n; ++i) {
    int u, v; cin >> u >> v;
    g[u].push_back(v);
    g[v].push_back(u);
  }
  for (int i = 1; i <= n; ++i) {
    if ((int)g[i].size() > 1) {
      root = i;
      break;
    }
  }
  pre_dfs(root, 0);
  cout << ((int)leaves.size() + 1) / 2 << '\n';
  sort(leaves.begin(), leaves.end(), [](const int &x, const int &y) -> bool {
    return tin[x] < tin[y];
  });
  for (int i = 0; i < ((int)leaves.size() + 1) / 2; ++i) {
    int rv = (int)leaves.size() - i - 1;
    if (i == rv) {
      rv = 0;
    }
    cout << leaves[i] << " " << leaves[rv] << '\n';
  }
}

signed main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);

  solve();

  return 0;
}

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