Pair sort

input/code.cpp: In function 'int main()':
input/code.cpp:50:21: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<std::pair<int, int> >::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   50 |   for (int i = 0; i < ans.size(); i++) {
      |                   ~~^~~~~~~~~~~~

#include <iostream>
#include <set>
#include <vector>

using namespace std;

const int N = 1e5 + 5;

int n;
int a[2 * N];
set<int> pos[N];
vector<pair<int, int> > ans;

void _swap(int u, int v) {
  ans.emplace_back(u, v);
  int val_u = a[u];
  pos[val_u].erase(u);
  pos[val_u].insert(v);
  int val_v = a[v];
  pos[val_v].erase(v);
  pos[val_v].insert(u);
  swap(a[u], a[v]);
}

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cin >> n;
  for (int i = 1; i <= 2 * n; i++) {
    cin >> a[i];
    pos[a[i]].insert(i);
  }
  for (int i = 1; i <= n - 1; i++) {
    int fi = *pos[i].begin();
    int se = *pos[i].rbegin();
    if (fi + 1 == se && fi & 1) {
      continue;
    }
    if (fi & 1) {
      _swap(se, fi + 1);
    } else if (se & 1) {
      _swap(fi, se + 1);
    } else if (fi % 2 == 0) {
      _swap(se, fi - 1);
    } else {
      _swap(fi, se - 1);
    }
  }
  cout << ans.size() << '\n';
  for (int i = 0; i < ans.size(); i++) {
    cout << ans[i].first << ' ' << ans[i].second << '\n';
  }
  return 0;
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: ACCEPTED

Test 7

Verdict: ACCEPTED

Test 8

Verdict: ACCEPTED

Test 9

Verdict: ACCEPTED

Test 10

Verdict: ACCEPTED

Test 11

Verdict: ACCEPTED

Test 12

Verdict: ACCEPTED

Test 13

Verdict: ACCEPTED

Test 14

Verdict: ACCEPTED