[LeetCode Python3]395. Longest Substring with At Least K Repeating Characters +递归

395. Longest Substring with At Least K Repeating Characters

class Solution:
    def longestSubstring(self, s: str, k: int) -> int:
        if len(s) < k:
            return 0
        for c in set(s):
            # 若某个字符c的个数大于0且小于k,那么包含它的子串必然不符合要求，
            # 所以用它将字符串分成若干个子串转化为子问题，递归求解
            if s.count(c) < k:
                return max(self.longestSubstring(t, k) for t in s.split(c))
        return len(s)