Pair sort

#include <bits/stdc++.h>
using namespace std;

int main(){
    ios::sync_with_stdio(false);
    cin.tie(NULL);
    
    int n;
    cin >> n;
    vector<int> arr(2*n);
    for(auto &x: arr) cin >> x;
    
    vector<vector<int>> positions(n+1, vector<int>());
    for(int i=0; i < 2*n; ++i){
        positions[arr[i]].push_back(i);
    }
    
    vector<pair<int, int>> swaps;

    for(int i=0; i < 2*n; i +=2){
        int first_num = arr[i];
        int second_pos;
        if(positions[first_num][0] == i){
            second_pos = positions[first_num][1];
        }
        else{
            second_pos = positions[first_num][0];
        }
        
        if(second_pos != i+1){
            int current_num_at_i1 = arr[i+1];
            swap(arr[i+1], arr[second_pos]);
            swaps.emplace_back(i+1 +1, second_pos +1);
            
            if(positions[current_num_at_i1][0] == second_pos){
                positions[current_num_at_i1][0] = i+1;
            }
            else{
                positions[current_num_at_i1][1] = i+1;
            }
            
            if(positions[first_num][0] == second_pos){
                positions[first_num][0] = i+1;
            }
            else{
                positions[first_num][1] = i+1;
            }
        }
    }
    
    cout << swaps.size() << "\n";
    for(auto &[x, y]: swaps){
        cout << x << " " << y << "\n";
    }
}

Test 1

Verdict: WRONG ANSWER

Test 2

Verdict: WRONG ANSWER

Test 3

Verdict: WRONG ANSWER

Test 4

Verdict: WRONG ANSWER

Test 5

Verdict: WRONG ANSWER

Test 6

Verdict: WRONG ANSWER

Test 7

Verdict: WRONG ANSWER

Test 8

Verdict: WRONG ANSWER

Test 9

Verdict: WRONG ANSWER

Test 10

Verdict: WRONG ANSWER

Test 11

Verdict: WRONG ANSWER

Test 12

Verdict: WRONG ANSWER

Test 13

Verdict: WRONG ANSWER

Test 14

Verdict: WRONG ANSWER