Merkkijono

#include <iostream>
#include <vector>

using namespace std;

int main()
{
	int n;
	cin >> n;
	vector<char> v, u;
	v.push_back('a');
	for (int i = 1; i < n; ++i) {
		u.push_back('a'+i);
		for (char c : v) {
			u.push_back(c);
			u.push_back('a'+i);
		}
		v = u;
		u.clear();
	}
	for (char c : v) cout << c;
	cout << endl;
}

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