Merkkijono

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class Main {

	public static void main(String[] args) throws IOException {
		BufferedReader bi = new BufferedReader(new InputStreamReader(System.in));
		int n = Integer.parseInt(bi.readLine());
		
		StringBuilder sb = new StringBuilder();
		for (int i = 0; i < n; i++) {
			char a = (char) (97 + i);
			for (int j = sb.length(); j >= 0; j -= 1) {
				sb.insert(j, a);
			}
		}
		System.out.println(sb.toString());
	}

}

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