题目
Given a string s and an integer k, return the length of the longest substring of s such that the frequency of each character in this substring is less than or equal to k.
解题思路
Sliding Window， 枚举答案子串中出现的不同字母的个数，然后维护当前有多少个不同的字母，以及有多少个字母出现的次数是k次，如果这两个数都是最开始假设的不同字母个数，就是一个可行解，更新结果
代码
class Solution:
def longestSubstring(self, s: str, k: int) -> int:
res = 0
for i in range(1, len(set(s)) + 1):
times = [0] * 26
l = r = ct = dif_ct = 0
while r < len(s):
ind = ord(s[r]) - ord('a')
times[ind] += 1
if times[ind] == 1:
dif_ct += 1
if times[ind] == k:
ct += 1
r += 1
while l < r and dif_ct > i:
ind = ord(s[l]) - ord('a')
if times[ind] == k:
ct -= 1
if times[ind] == 1:
dif_ct -= 1
times[ind] -= 1
l += 1
if dif_ct == ct == i:
res = max(res, r - l)
return res
