Longest substring with at most two distinct characters

We will use a sliding window to solve this problem. The window will be a substring of the input string. The window will be valid if it has at most two distinct characters. We will keep track of the maximum length of the valid window.

class Solution:
    def lengthOfLongestSubstringTwoDistinct(self, s: str) -> int:
        count = collections.defaultdict(int)
        i, res = 0, 0
        for j, ch in enumerate(s):
            count[ch] += 1
            while len(count) > 2:
                count[s[i]] -= 1
                if count[s[i]] == 0:
                    del count[s[i]]
                i += 1
            res = max(res, j-i+1)    
        return res

Time complexity: O(n)
Space complexity: O(n)