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

        """
        At each step, right pointer points to a locations where we
        are trying to calculate length of longest string ending there.
        When we are iterating over the character corresponding to right pointer,
        we might have added one of the duplicate characters already in the hashset
        so we have to remove characters from left untill duplicate of s[right] is removed

        Think of it this way:
        how will you calcuate the longest substring ending at aright pointer
        """

        left = 0
        hashset = set()
        ans = 0

        for right in range(len(s)):
            while(s[right] in hashset):
                hashset.remove(s[left])
                left=left+1

            hashset.add(s[right])
            ans = max(ans, right - left + 1)

        return ans