Fragile network

input/code.cpp: In function 'int main()':
input/code.cpp:39:5: error: 'sort' was not declared in this scope; did you mean 'short'?
   39 |     sort(g[i].begin(), g[i].end());
      |     ^~~~
      |     short

#include <iostream>
#include <vector>

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;
}