Pair sort

input/code.cpp: In function 'int main()':
input/code.cpp:35:23: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<std::pair<int, int> >::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   35 |     for (int i = 0; i < swaps.size(); ++i) {
      |                     ~~^~~~~~~~~~~~~~

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

int main() {
    int n;
    cin >> n;
    vector<int> numbers(2* n);
    for (int i = 0; i < 2 * n; ++i) {
        cin >> numbers[i];
    }

    unordered_map<int, vector<int>> pos_map;
    for (int i = 0; i < 2 *n; ++i) {
        pos_map[numbers[i]].push_back(i);
    }

    vector<pair<int, int>> swaps;
    for (int i = 0; i < 2 * n; i += 2) {
        int num = numbers[i];
        if (numbers[i +1] != num) {
            int pos = pos_map[num][1];
            swaps.push_back(make_pair(i + 1, pos));
            swap(numbers[i + 1], numbers[pos]);
            pos_map[numbers[pos]].pop_back();
            pos_map[num][1] = i+ 1;
        } 
        else {
            pos_map[num].erase(pos_map[num].begin());
        }
    }

    cout << swaps.size() << endl;
    for (int i = 0; i < swaps.size(); ++i) {
        cout << swaps[i].first + 1 << " " << swaps[i].second + 1 << endl;
    }
}

Test 1

Verdict: RUNTIME ERROR

Test 2

Verdict: RUNTIME ERROR

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