I made this program to train me for a competition. The hypothetical question I’ve used was the one in the comments near the top of the code. It was fun making the code; however, it felt like it took too long for such a simple code. Is there any way to make this simpler or better?

import java.util.*;  public class CopsAndRobbers {  public static void main(String[] args) {     //The cops need help finding who is the most probable criminal to commit a crime     //Witnesses tell them multiple different people, making it hard to find the criminal     //Use your knowledge of coding and the one-letter identifications provided for the criminals     //To find who most likely did it and print.      TreeSet<String> done = new TreeSet<String>();     ArrayList<String> all = new ArrayList<String>();     ArrayList<Integer> num = new ArrayList<Integer>();     ArrayList<String> some = new ArrayList<String>();     @SuppressWarnings("resource")     Scanner sc = new Scanner(System.in);     String input = sc.nextLine();      for(int i = 0; i < input.length(); i++)     {         done.add(String.valueOf(input.charAt(i)));         all.add(String.valueOf(input.charAt(i)));     }     int amount = done.size();     for(int i = 0; i < amount; i++)     {         String letter = done.first();         int tempNum = 0;         for(int j = 0; j < all.size(); j++)         {             if(all.get(j).equals(letter))             {                 tempNum++;             }         }         num.add(tempNum);         some.add(letter);         done.remove(letter);     }     int highest = 0;     int overallIndex = 0;     for(int i = 0; i < num.size(); i++)     {         if(num.get(i) > highest)         {             highest = num.get(i);             overallIndex = i;         }     }     System.out.println("The most likely criminal is " + some.get(overallIndex) + " with " + highest + " witnesses."); }  }