Verification error definetely

package com.codegym.task.task39.task3901; import java.io.BufferedReader; import java.io.IOException; import java.io.InputStreamReader; import java.util.HashMap; /* Unique substrings */ public class Solution { public static void main(String[] args) throws IOException { BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(System.in)); System.out.println("Please enter your string: "); String s = bufferedReader.readLine(); System.out.println("The longest unique substring length is: \n" + lengthOfLongestUniqueSubstring(s)); } public static int lengthOfLongestUniqueSubstring(String s) { if(s.length() == 0 || s.equals(null)) return 0; HashMap<Integer,Integer> mk=new HashMap<Integer,Integer>(); int greatest=0;//length of longest repeating substring int currentIndex=0; //starting index of longest non-repeating substring int index=0; //System.out.println(s); while(index<s.length()) { //System.out.println("Letter is "+s.charAt(index)+", greatest is "+greatest+", currentIndex is "+currentIndex); int key=(int)s.charAt(index); if(mk.containsKey(key)) { if(mk.get(key)>=currentIndex) { //check if this letter is really applicable(occurs at or after the beginning of currentLongestSubstring) if(index - currentIndex > greatest)//update greatest if new Longest Substring is found greatest=(index-currentIndex); currentIndex=mk.get(key)+1; } mk.put(key, index); }else { mk.put(key,index); } index++; } if(s.length()-currentIndex > greatest) greatest = (s.length()-currentIndex);//if greatest length is at end of string return greatest; } }