Merkkijono

import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		String letter = "abcdefghijklmnopqrstuvwxyz";
		
        Scanner input = new Scanner(System.in);
        
        int a = input.nextInt(); //length
        String fString = "";
        
        for(int i = 0; i < a; i++) {
        	String currentLetter = letter.substring(i, i+1);
        	String newString = "";
        	int l = fString.length();
        	int it = 0;
        	newString += currentLetter;
        	//System.out.println("Free ram:" + Runtime.getRuntime().freeMemory());
        	while(it<l) {
        		newString += fString.substring(it, it+1);
        		newString += currentLetter;
        		it++;
        	}
        	fString = newString; //seemingly the failure point. too long?
        }
        
        System.out.println(fString);
        
        input.close();
    }
	
}

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