Merkkijono

#include <bits/stdc++.h>

typedef long long ll;

#define M 1000000007
#define N (1 << 18)

using namespace std;

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    int n;
    cin >> n;
    string s = "a";
    string ab = "abcdefghijklmnop";

    for (int i = 1; i < n; ++i) {
        string nw = ""; nw += ab[i];
        for (int j = 0; j < (int)s.size(); ++j) {
            nw += s[j];
            nw += ab[i];
        }
        s = nw;
    }
    cout << s << 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

Test 15

Verdict: ACCEPTED

Test 16

Verdict: ACCEPTED