Looks like the provided solution is not perfect, when we found a duplicate char - we should jump the left pointer to the prev duplicate char position + 1 instead of just doing start += 1

class Solution2:
    def lengthOfLongestSubstring(self, s: str) -> int:
        # key: char
        # value: index of char in s
        m = {}
        max_length = 0  # init to 0, to handle empty string cases
        left = 0
        for right in range(len(s)):
            c = s[right]

            # if found a duplicate, and it's in the current window,
            # move left to the next non-duplicate position
            if c in m and m[c] >= left:
                left = m[c] + 1
            m[c] = right

            cur_length = right - left + 1
            max_length = max(max_length, cur_length)

        return max_length