Pair sort

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

int main() {
	int n;
	cin >> n;

	int a[(int)2e5];
	int b[(int)2e5+2];
	for (int i=2; i<2*(n+1); i++) {
		b[i] = -1;
	}
	for (int i=0; i<2*n; i++) {
		int temp;
		cin >> temp;
		a[i] = temp;
		if (b[2*temp] == -1)
			b[2*temp] = i;
		else
			b[2*temp+1] = i; 
	}
	vector<int> swaps;
	int swapn = 0;

	for (int i=1; i<n; i++) {
		int si = b[2*i];
		int ei = b[2*i+1];
		int fhi, foi;
		if (si % 2 == 0) {
			fhi = ei;
			foi = si+1;
		} else if (ei % 2 == 0) {
			fhi = si;
			foi = ei+1;
		} else {
			fhi = si;
			foi = ei-1;
		}
		int swap = a[foi];
		int swapi;
		if (b[2*swap] == foi) 
			swapi = 2*swap;
		else 
			swapi = 2*swap+1;
		
		b[swapi] = fhi;
		a[fhi] = swap;
		a[foi] = i;
		
		swaps.push_back(fhi+1);
		swaps.push_back(foi+1);
		swapn++;
	}

	cout << swapn << '\n';
	for (int i=0; i<swapn; i++) {
		cout << swaps[2*i] << " " << swaps[2*i+1] << '\n';
	}
	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