class Solution:
    def lengthOfLongestSubstring(self, s: str) -> int:
        if not s:
            return 0

        fast = 0
        window_element = set()
        max_length = 0
        for slow in range(len(s)):
            while fast < len(s) and s[fast] not in window_element:
                window_element.add(s[fast])
                max_length = max(max_length, len(window_element))
                fast += 1            
            window_element.remove(s[slow])

        return max_length

class Solution:
    def lengthOfLongestSubstring(self, s: str) -> int:

        if len(s) == 0:
            ans = 0
        else:
            ans = 0
            queue_list = []
            l = 0 # left pointer

            for r in range(len(s)): # s[r], get one new word
                # found duplicate in sliding window, remove until duplicate not found
                while s[r] in queue_list: 
                    queue_list.pop(0)
                    l += 1
                # not found or all duplicate removed, push new word s[r]
                queue_list.append(s[r]) 
                # update answer
                ans = max(ans, r-l+1)     
        return ans