'''
Description: 
Autor: Au3C2
Date: 2021-02-27 10:33:43
LastEditors: Au3C2
LastEditTime: 2021-02-27 10:34:37
'''
class Solution:
    def longestSubstring(self, s: str, k: int) -> int:
        stack = [s]; ans = 0
        while stack:
            s = stack.pop()
            for c in set(s): # 遍历s中的唯一字母
                if s.count(c) < k:
                    temp = []
                    for t in s.split(c):
                        if t:
                            temp.append(t)
                    stack.extend(temp)
                    break
            else:
                ans = max(ans, len(s))
        return ans
    
# 递归，中等，每日一题
# 这解法一点也不递归？
# https://leetcode-cn.com/problems/longest-substring-with-at-least-k-repeating-characters/