LeetCode: Maximum Number of Vowels in a Substring of Given Length

Find maximum number of vowels in any substring of given length k.

Posted on 4 Dec 2025 LeetCode Python

Difficulty: Medium

This problem is nearly identical to Maximum Average Subarray I, but instead of calculating averages, we are counting vowels in substrings.

Solution

Calculate the number of vowels in the first k elements.

Use a sliding window approach to iterate through the array starting from index k. For each new element added to the window, add to the vowel count if the i-th element is a vowel, and subtract from the vowel count if the i - k-th element is a vowel. Update the maximum vowel count found so far.

Runtime

47 ms | Beats 95.01%

class Solution:
    def maxVowels(self, s: str, k: int) -> int:
        vowels = {"a", "e", "i", "o", "u"}

        count_vowels = 0

        for i in range(k):
            if s[i] in vowels:
                count_vowels += 1

        max_vowels = count_vowels

        for i in range(k, len(s)):
            if s[i] in vowels:
                count_vowels += 1
            if s[i - k] in vowels:
                count_vowels -= 1
            if count_vowels > max_vowels:
                max_vowels = count_vowels

        return max_vowels