3

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

        l = 0
        r = 0
        memo = set()
        res = 0

        while r < len(s):
            if s[r] not in memo:
                memo.add(s[r])
                r += 1
                res = max(res, r - l)
            else:
                memo.remove(s[l])
                l += 1

        return res