findMaxSubString with adj chars diff less than k

import java.util.*;
public class Main
{
    private static String findMaxSubString(String s,int k){
        if(s.length()==0) return "";

        char prev = s.charAt(0);
        int resLen = 1;
        int currLen = 1;
        int endingInd = 0;
        for(int i =1;i<s.length();i++){
            char curr = s.charAt(i);

            if(Math.abs(curr-prev)<=k){
                currLen+=1;
            }else{
                currLen=1;
            }

            if(currLen>resLen){
                resLen = currLen;
                endingInd=i;
            }
            prev = curr;
        }
        int startingInd = endingInd-resLen+1;
        return s.substring(startingInd,endingInd+1);
    }
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String s = sc.nextLine();
        int k = sc.nextInt();

        System.out.println(findMaxSubString(s,k));
    }
}