Maximum number of vowel letters in any substring of string 's' with length 'k'

window.py

class Solution(object):
    def maxVowels(self, s, k):
        """
        :type s: str
        :type k: int
        :rtype: int
        """
        maxCount = 0
        lastCount = -1
        lastSubstrFirstLetterIsVowel = False
        isVowel = lambda letter: letter in ['a','e','i','o','u']
        for i in range(len(s)):
            if i + k > len(s):
                break
            vowCount = 0
            if lastCount == -1:
                substr = s[i:i+k]
                vowCount = len(filter(isVowel, substr))
            else:
                vowCount = lastCount - (1 if lastSubstrFirstLetterIsVowel else 0) + isVowel(s[i+k-1])
            lastSubstrFirstLetterIsVowel = isVowel(s[i])
            maxCount = max(vowCount, maxCount)
            lastCount = vowCount

        return maxCount