Fragile network

#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

const int N = 1e5 + 5;

int n;
vector<int> g[N];
vector<pair<int, int> > ans;
bool vis[N];
vector<int> l;

void dfs(int u) {
  bool f = true;
  vis[u] = true;
  for (int v : g[u]) {
    if (!vis[v]) {
      f = false;
      dfs(v);
    }
  }
  if (f) {
    l.emplace_back(u);
  }
}

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cin >> n;
  for (int i = 1; i <= n - 1; i++) {
    int a, b;
    cin >> a >> b;
    g[a].emplace_back(b);
    g[b].emplace_back(a);
  }
  for (int i = 1; i <= n; i++) {
    sort(g[i].begin(), g[i].end());
  }
  dfs(1);
  int x = 0, y = (int) l.size() - 1;
  while (x <= y) {
    if (x == y) {
      ans.emplace_back(l[x], l.back());
      break;
    }
    ans.emplace_back(l[x], l[y]);
    x++;
    y--;
  }
  cout << ans.size() << '\n';
  for (auto x : ans) {
    cout << x.first << ' ' << x.second << '\n';
  }
  return 0;
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: WRONG ANSWER

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: ACCEPTED

Test 7

Verdict: WRONG ANSWER

Test 8

Verdict: ACCEPTED

Test 9

Verdict: WRONG ANSWER

Test 10

Verdict: WRONG ANSWER

Test 11

Verdict: WRONG ANSWER

Test 12

Verdict: ACCEPTED

Test 13

Verdict: ACCEPTED

Test 14

Verdict: WRONG ANSWER

Test 15

Verdict: ACCEPTED

Test 16

Verdict: ACCEPTED

Test 17

Verdict: ACCEPTED

Test 18

Verdict: ACCEPTED

Test 19

Verdict: ACCEPTED

Test 20

Verdict: ACCEPTED

Test 21

Verdict: ACCEPTED