Find the Longest Substring without Repeating Characters - Java

I was trying to solve a leetcode question but my solution is not returning the correct value for one of the test cases. I wanted to implement the two-pointer technique for this solution (if im using the technique wrong, advice is welcome on explaining the proper way to do it). The details are below:

Leetcode Question Title:

> Longest Substring Without Repeating Characters.

Question:

> Given a string, find the length of the longest substring without repeating characters. ( return int )

My Solution:

    public int lengthOfLongestSubstring(String s) {
		//place holder for me to create a "sub-string"
        String sub = "";
		
		//counter variable to count the characters in the substring
        int count = 0;
        int maxCount = 0;
        
		//my attempt at a TWO POINTER TECHNIQUE
        //pointers - to determine when the loop should break
        int head = 0;
        int tail = s.length();
		
		//this variable was intended to be used with the "indexOf" method, as the "start from" index...
        int index = 0;
        
        while(head < tail){
			//check if the next character in the string was already added to my substring.
            int val = sub.indexOf(s.charAt(head), index);
			
			//we proceed if it is -1 because this means the substring didnt previously contain that character
            if(val == -1){
				//added character to my substring
                sub+= s.charAt(head);
                count++;
                head++;
            } else {
				//reset data to default conditions, while continuing at the "head index" and check the rest of the substring
                count = 0;
                sub =  "";
            }
			//determine what the length of the longest substring.
            maxCount = count > maxCount ? count : maxCount;
        }
        return maxCount;
    }

I passed the test cases:

> "" = expect 0
> " " = expect 1
> "abcabcbb" = expect 3
> "bbbbb" = expect 1
> "pwwkew" = expect 3
> "aab" = expect 2

I failed test cases:

> "dvdgd" = expect 3, but got 2
> "dvjgdeds" = expect 5, but got 4
> "ddvbgdaeds" = expect 6, but got 4

Possible Issues:

> I believe the issue occurred because it moves pass the index with "v", and then processes the substring "dg". So I attempted to change the solution, but fixing that issue caused all my other cases to return errors so I figured that attempt at a fix should be tossed out.

What I have also tried:

> I also attempted to manipulate the "indexOf" method to change the start index, whenever the character was found in the string. This however generated an infinite loop.

I have been trying to solve this problem for a few hours and I am stomped. So any help would be appreciated greatly. And please be detailed with your explanation if you can, I am very new to DSA and programming, thank you greatly. If more information from me is needed, please let me know am I will try to answer.

Well, here you go:

public static int lengthOfLongestSubstring(String s) {
    //place holder for me to create a "sub-string"
    String sub = "";
    
    //counter variable to count the characters in the substring
    int count = 0;
    int maxCount = 0;
    
    //my attempt at a TWO POINTER TECHNIQUE
    //pointers - to determine when the loop should break
    int head = 0;
    int tail = s.length();
    
    //this variable shows where to start from 
    int index = 0;
    
    while(head < tail){
        //check if the next character in the string was already added to my substring.
        int val = sub.indexOf(s.charAt(head)); //search whole substing
        
        //we proceed if it is -1 because this means the substring didnt previously contain that character
        if(val == -1){
            //added character to my substring
            sub+= s.charAt(head);
            count++;
            head++;
            //determine what the length of the longest substring.
            maxCount = count > maxCount ? count : maxCount;
            System.out.println(sub); //let's see what we got so far
        } else {
            //reset data to default conditions, while continuing at the "head index" and check the rest of the substring
            count = 0;
            sub =  "";
            head=index+1; //begin from the next letter 
            index++; //move
        }
        
        
    }
    return maxCount;
}