Longest sub string in a string - LeetCode

longestSubString

/*
Given a string, find the length of the longest substring without repeating characters. 
For example, the longest substring without repeating letters for "abcabcbb" is "abc", 
which the length is 3. For "bbbbb" the longest substring is "b", with the length of 1.
*/
public class Solution {
    public int lengthOfLongestSubstring(String s) {
        if(s == null || s.length() == 0) return 0; 
        
        // Time Complexity = O(n*2) = O(n), Space Complexity = O(n)
        int max = 0; 
        int slow = 0; 
        int fast = 0; 
        HashSet<Character> set = new HashSet<Character>();
        
        while(fast < s.length()) {
            if(set.contains(s.charAt(fast))) {
                max = Math.max(max, fast - slow);
                while(s.charAt(slow) != s.charAt(fast)) {
                    set.remove(s.charAt(slow));
                    slow++;
                }
                slow++;
            } else set.add(s.charAt(fast));
            fast++;
        }
        max = Math.max(max, fast - slow);
        return max;
    }
}