340. Longest Substring with At Most K Distinct Characters

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

        if k == 0:
            return 0


        M = defaultdict(int)

        ss = set()

        res = 0

        l = - 1
        for i in range(len(s)):

            M[s[i]] += 1

            if M[s[i]] == 1:
                ss.add(s[i])

            while len(ss) > k:

                l += 1
                M[s[l]] -= 1

                if M[s[l]] == 0:
                    ss.remove(s[l])

            res = max(res, i - l)

        return res