String Mix

import java.util.*;

public class Mixing {

    public static String mix(String s1, String s2) {
      // your code
      /*String s1A = s1.replaceAll("[^a-z]", "");
      String s2A = s2.replaceAll("[^a-z]", "");*/
      int[][] count = new int[2][26];
      ArrayList<String> list = new ArrayList<>();

      int index = 0;
      for (char c = 'a'; c <= 'z'; c++){
        final char C = c;
        count[0][index] = (int) s1.chars().filter(i -> i == C).count();
        count[1][index] = (int) s2.chars().filter(i -> i == C).count();
        index++;
      }

      for (int i = 0; i < 26; i++){
        if (count[0][i] > 1 || count[1][i] > 1){
          int j = i+97;
          char c = (char)j;
          String s = count[0][i] > count[1][i] ? ("1:"+String.join("", Collections.nCopies(count[0][i], String.valueOf(c)))) :
                    count[1][i] > count[0][i] ? ("2:"+String.join("", Collections.nCopies(count[1][i], String.valueOf(c)))) :
                    ("=:"+String.join("", Collections.nCopies(count[1][i], String.valueOf(c))));
          list.add(s);
        }
      }

      //Arrays.sort(rank, Comparator.comparingInt(String::length));
      Collections.sort(list, new LengthFirstComparator());

      StringBuffer sb = new StringBuffer("");
      for (String s : list){
        if (!s.equals("")){
          sb.append(s);
          sb.append("/");
        }
      }
      if (sb.length()>0){
        sb.deleteCharAt(sb.length()-1);
      }
      return sb.toString();

    }
  static class LengthFirstComparator implements Comparator<String> {
    @Override
    public int compare(String o1, String o2) {
        if (o1.length()!=o2.length()) {
            return o2.length()-o1.length(); //overflow impossible since lengths are non-negative
        }
        return o1.compareTo(o2);
    }
}
}