class Solution:
    def lengthOfLongestSubstring(self, s: str):
        start = -1
        max   = 0
        d ={}
        
        for i in range(len(s)):
            if s[i] in d and d[s[i]] > start:
                start = d[s[i]]
                d[s[i]] = i
            else:
                d[s[i]] = i
                if i - start > max:
                    max = i - start
        return max