class Solution:
    def lengthOfLongestSubstring(self, s: str) -> int:
        """
        >>> Solution().lengthOfLongestSubstring("abcabcbb")
        3
        >>> Solution().lengthOfLongestSubstring("bbbbb")
        1
        >>> Solution().lengthOfLongestSubstring("pwwkew")
        3
        """
        if s == "":
            return 0

        beginning = 0
        end = 1
        longest = 0
        while end < len(s):
            p = s[beginning:end]
            if s[end] in s[beginning:end]:
                if end - beginning > longest:
                    longest = end - beginning
                beginning += 1
            else:
                end += 1
        
        if end - beginning > longest:
            longest = end - beginning
        return longest


if __name__ == "__main__":
    import doctest
    doctest.testmod()