395. Longest Substring with At Least K Repeating Characters

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Not the common pattern. We use slide window with restricted condition.

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

        n = len(set(s))

        res = 0

        def helper(pairN):
            M = defaultdict(int)
            l = 0
            S = set()
            count = 0
            res = 0
            for i in range(len(s)):
                M[s[i]] += 1
                S.add(s[i])

                if M[s[i]] == k:
                    count += 1

                while len(S) > pairN:
                    M[s[l]] -= 1
                    if M[s[l]] == k - 1:
                        count -= 1
                    if M[s[l]] == 0:
                        S.remove(s[l])
                    l += 1
            
                if len(S) == pairN and count == pairN:
                    res = max(res, i - l + 1)

            return res

        for i in range(1, n+1):
            res = max(res, helper(i))

        return res