LeetCode 395. Longest Substring with At Least K Repeating Characters

Problem Statement

(Source) Find the length of the longest substring T of a given string (consists of lowercase letters only) such that every character in T appears no less than k times.

Example 1:

Input:
s = "aaabb", k = 3

Output:
3

The longest substring is "aaa", as 'a' is repeated 3 times.

Example 2:

Input:
s = "ababbc", k = 2

Output:
5

The longest substring is "ababb", as 'a' is repeated 2 times and 'b' is repeated 3 times.

Solution 1

class Solution(object):
    def longestSubstring(self, s, k):
        """
        :type s: str
        :type k: int
        :rtype: int
        """
        if len(s) < k:
            return 0

        ch = min(set(s), key=s.count)
        if s.count(ch) >= k:
            return len(s)
        return max([self.longestSubstring(t, k) for t in s.split(ch)])

Solution 2

class Solution(object):
    def longestSubstring(self, s, k):
        """
        :type s: str
        :type k: int
        :rtype: int
        """
        from collections import Counter

        cnt = Counter(s)
        if not s or cnt.most_common(1)[0][1] < k:
            return 0
        elif cnt.most_common()[-1][1] >= k:
            return len(s)
        else:
            return max(self.longestSubstring(ss, k) for ss in s.split(cnt.most_common()[-1][0]))