Pair sort

#include <iostream>
#include <vector>
#include <algorithm>
#include <string>
#include <set>

using namespace std;

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

    // freopen("input.txt", "r", stdin); // TODO: REMOVE THIS YOU STUPID ****

    int n;
    cin >> n;
    int m = 2 * n;
    vector<int> a(m);
    for (int i = 0; i < m; ++i) cin >> a[i];

    vector< set<int> > pos(n + 1);
    for (int i = 0; i < m; ++i) pos[a[i]].insert(i);

    vector<pair<int,int>> swaps;
    for (int i = 0; i < m; i += 2) {
        int x = a[i];
        pos[x].erase(i);
        int j = *pos[x].begin();
        if (j == i + 1) {
            // already paired
            pos[x].erase(j);
            continue;
        }
        // swap a[i+1] and a[j]
        int y = a[i+1];
        pos[y].erase(i+1);
        pos[y].insert(j);
        pos[x].erase(j);
        swap(a[i+1], a[j]);
        swaps.emplace_back(i+2, j+1);
    }

    cout << swaps.size() << endl;
    for (auto &p : swaps) cout << p.first << " " << p.second << endl;
    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