Pair sort

#include <iostream>
#include <string>
#include <vector>
using namespace std;

int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);

  int n;
  cin >> n;
  vector<int> a(2 * n), first(n + 1, -1), partner(2 * n);
  for (int i = 0; i < 2 * n; i++) {
    cin >> a[i];
    if (first[a[i]] < 0) {
      first[a[i]] = i;
    } else {
      partner[i] = first[a[i]];
      partner[first[a[i]]] = i;
    }
  }

  string out;
  int k = 0;
  for (int i = 0; i + 1 < 2 * n; i += 2) {
    int j = partner[i];
    if (j == i + 1) continue;
    int p = partner[i + 1];
    swap(a[i + 1], a[j]);
    partner[p] = j;
    partner[j] = p;
    partner[i] = i + 1;
    partner[i + 1] = i;
    out += to_string(i + 2) + ' ' + to_string(j + 1) + '\n';
    k++;
  }

  cout << k << '\n' << out;
}

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