Merkkijono

import java.util.HashMap;
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 = "";
        HashMap<Integer, String> map = new HashMap<>();
        
        for(int i = 0; i < a; i++) {
        	String currentLetter = letter.substring(i, i+1);
        	//System.out.println("starting " + i + " letter " + currentLetter);
        	String newString = "";
        	int l = 0;
        	for(String entry : map.values()) {
        		l += entry.length();
        	}
        	int it = 0;
        	newString += currentLetter;
        	//System.out.println("map size " + map.size() + " length " + l + " new " + newString + " start of new " + fString);
        	int sector = 0;
        	int rSector = 0;
        	HashMap<Integer, String> readMap = new HashMap<>(map);
        	fString = readMap.get(sector);
        	map.clear();
        	while(it<l) {
        		if(it*2+1 > 16000+(sector*16000)) {
        			map.put(sector, newString);
        			sector++;
        			newString = "";
        		}
        		if(it > 16000+(rSector*16000)) {
        			fString = readMap.get(rSector);
        			rSector++;
        		}
        		newString += fString.substring(it-(rSector*16000), it+1-(rSector*16000));
        		newString += currentLetter;
        		it++;
        		//System.out.println("i " + i + " it " + it + " l " + l);
        	}
        	map.put(sector, newString);
        	//System.out.println("out ");
        	//fString = newString; //seemingly the failure point. too long to save
        	//System.out.println("mid " + fString);
        }
        //System.out.println("Ending with map size of " + map.size());
        //System.out.println("Sector1 " + map.get(0));
        //System.out.println("FINAL ");
        for(String entry : map.values()) {
        	System.out.print(entry);
    	}
        //System.out.println("close.");
        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: WRONG ANSWER

Test 16

Verdict: WRONG ANSWER