Longest Nice String

class Solution {
    public String longestNiceSubstring(String s) {
       String res = "";

        int lenOfWholeString = s.length();
       for (int start = 0; start < lenOfWholeString - 1; start++) {
           // if the remaning chars less than the res len found till now , no need to continue the iteration 

           int remaningChars = lenOfWholeString - start;
           if (remaningChars <= res.length()) {
               break;
           }
           // here we are sure that the remaning chars count > the length of last res we found 


           HashSet<Character> lowerSet = new HashSet<>();
           HashSet<Character> upperConvertedToLowerSet = new HashSet<>();
           
           // iterate over all chars starting from "start"
            // - if its lower , put it into the lowerSet
            // - if it is upper , convert it into lower and add it into the upperConvertedToLowerSet 
            //  - check if lowerSet equals upperConvertedToLowerSet , if so , update the solution

           for (int curr = start; curr < lenOfWholeString; curr++) {
                if(Character.isLowerCase(s.charAt(curr))) {
                      lowerSet.add(s.charAt(curr));
                } else {
                    upperConvertedToLowerSet.add(Character.toLowerCase(s.charAt(curr)));
                }
                if (lowerSet.equals(upperConvertedToLowerSet)) {
                    int currSolutionLen = curr - start + 1;
                    if (currSolutionLen > res.length()) {
                        res = s.substring(start, curr + 1);
                    }
                }
           }
       }

       return res;
    }


    /*
      - i
    
    **/
    
    
    
     
}