Fragile network

input/code.cpp: In function 'int main()':
input/code.cpp:46:36: warning: comparison of integer expressions of different signedness: 'std::vector<int>::size_type' {aka 'long unsigned int'} and 'int' [-Wsign-compare]
   46 |   for (int i = 0; i + l.size() / 2 < (int) l.size(); i++) {
      |                   ~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~

#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);
  if (l.size() & 1) {
    l.emplace_back(l.back());
  }
  for (int i = 0; i + l.size() / 2 < (int) l.size(); i++) {
    ans.emplace_back(l[i], l[i + l.size() / 2]);
  }
  std::cout << ans.size() << '\n';
  for (auto x : ans) {
    std::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: ACCEPTED

Test 12

Verdict: ACCEPTED

Test 13

Verdict: ACCEPTED

Test 14

Verdict: ACCEPTED

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