valid substring with no char pairs difference greater than k

import java.util.*;
// Online Java Compiler
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/*
test case -

abccadfdd
3

expected ans -
abccad
*/

public class Main {

  public static boolean checkIfValidSubString(int k,LinkedHashSet<Character>  charSet){
    int difference = Math.abs(Collections.max(charSet) - Collections.min(charSet));
    return difference<=k;
  }
  public static String getMaxValidSubString(String s,int k){
    int left =0,right = 0;
    int mainLeft =0,mainRight = 0;

    HashMap<Character, Integer> frequencyMap = new HashMap<>();
    LinkedHashSet<Character> charSet = new LinkedHashSet<>();

    while(left<=right && right<s.length()){
      char currChar = s.charAt(right);
      int currCharFreq = frequencyMap.getOrDefault(currChar, 0);
      if(currCharFreq==0) {
        frequencyMap.put(currChar,1);
      }else{
        frequencyMap.put(currChar,currCharFreq+1);
      }
      charSet.add(currChar);
      while(left<=right && !checkIfValidSubString(k,charSet) ){
        char leftChar = s.charAt(left);
        int leftCharFreq = frequencyMap.get(leftChar);
        if(leftCharFreq<=1){
          charSet.remove(leftChar);
        }
        frequencyMap.put(leftChar,leftCharFreq-1);
        left+=1;
      }

      if(left<s.length() && ((mainRight-mainLeft)<(right-left))){
        mainRight = right;
        mainLeft = left;
      }
      right+=1;
    }

    return s.substring(mainLeft,mainRight+1);
  }
  public static void main(String[] args) {
      Scanner sc = new Scanner(System.in);
      String s = sc.nextLine();
      int k = sc.nextInt();
      System.out.println(getMaxValidSubString(s,k));
  }
}