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

        #TC: O(len(s))
        #SC: O(1)

        if len(s) == 0:
            return 0

        left = 0
        right = 1

        Dict = collections.defaultdict(int)
        Dict[s[0]] = 1

        res = 1

        while(right < len(s)):
            Dict[s[right]] += 1

            if len(Dict) <= 2:
                Len = right - left + 1
                res = max(Len, res)
            else:
                while(left < right):
                    left += 1
                    Dict[s[left-1]] -= 1
                    if Dict[s[left-1]] == 0:
                        del Dict[s[left-1]]

                    if len(Dict) <= 2:
                        Len = right - left + 1
                        res = max(Len, res)
                        break
            right += 1

        return res