Pair sort

input/code.cpp: In function 'int main()':
input/code.cpp:31:17: warning: unused variable 'current_num_at_i1' [-Wunused-variable]
   31 |             int current_num_at_i1 = arr[i+1];
      |                 ^~~~~~~~~~~~~~~~~

#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 +2, second_pos +1); 
            int swapped_num = arr[i+1];
            int current_num_at_second_pos = arr[second_pos];
            
            for(auto &pos : positions[swapped_num]){
                if(pos == second_pos){
                    pos = i+1;
                    break;
                }
            }
            
            for(auto &pos : positions[current_num_at_second_pos]){
                if(pos == i+1){
                    pos = second_pos;
                    break;
                }
            }
        }
    }
    cout << swaps.size() << "\n";
    for(auto &[x, y]: swaps){
        cout << x << " " << y << "\n";
    }
}

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