Merkkijono

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

int main()
{
    int n, size;
    cin >> n;
    n--;
    char letters[] = {'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q'};
    vector<char> arr = {'a'};
    auto it = arr.begin();
    for (int i = 0; i < n; i++)
    {
        size = arr.size();
        for (int j = 0; j <= size; j++)
        {
            it = arr.insert(arr.begin() + j * 2, letters[i]);
        }
    }
    for (auto x : arr)
        cout << x;
    cout << "\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

Test 15

Verdict: ACCEPTED

Test 16

Verdict: ACCEPTED