395. Longest Substring with At Least K Repeating Characters

以次数不满k的字符分割字符串 因为此类字符可能会有多个 所以递归调用自身
注意为了避免重复判定分割字符 所以在循环中返回
比如s = “abcabbdaababbcaabd”, k = 3 判定的是以下字符串
abcabb
aababbcaab

bc
bb
aababb
aab

b

class Solution(object):
    def longestSubstring(self, s, k):
        """
        :type s: str
        :type k: int
        :rtype: int
        """
        if len(s) < k:
            return 0 
        for c in set(s):
            if s.count(c) < k:
                return max(self.longestSubstring(t,k) for t in s.split(c))
        return len(s)