例

Input: 
    s = "pwwkew"
Output: 
    3
Explanation: 
    The answer is "wke", with the length of 3.

CODE

def lengthOfLongestSubstring(s):
    left, max_length = 0, 0
    substring = set()
    for right in range(len(s)):
        if s[right] not in substring:
            substring.add(s[right])
            max_length = max(max_length, right - left + 1)
        else:
            # ifではなくwhile
            while s[right] in substring:
                substring.remove(s[left])
                left += 1
            # 結局、右要素をaddする
            substring.add(s[right])
            
    return max_length

set()で初期化ができて、add・removeで処理する。