Given a string s and an integer k, return the maximum number of vowel letters in any substring of s of length k.

Note: Vowel letters in English are 'a', 'e', 'i', 'o', and 'u'.

Constraints:

• $1 \leq$ s.length $\leq 10^5$

• s consists of lowercase English letters.

• $1 \leq$ k $\leq$ s.length

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