Basic example
Input
s = "abciiidef", k = 3
Output
3
The substring "iii" contains 3 vowels which is the maximum for any window of size 3.
Full lesson preview
Find the maximum vowels in a substring of size K
Find the maximum number of vowels contained in any substring of length K using a sliding window.
Python practice6 minStrings – Sliding Window & FrequencyBeginnerLast updated March 25, 2026
Problem statement
Given a string s and an integer k, return the maximum number of vowels in any substring of s with length k. Vowels are 'a', 'e', 'i', 'o', 'u' (lowercase). If k is 0 return 0. If k is greater than the length of s, return 0.
Task
Implement an efficient sliding-window algorithm to count vowels in fixed-size substrings and return the maximum count.
Examples
Input format
Function max_vowels(s: str, k: int) -> int
Output format
Return an integer representing the maximum vowel count in any substring of length k.
Constraints
0 <= k <= len(s) <= 10^5. Expected O(n) time and O(1) extra space.
Samples
Sample 1
Input
s = "aeiou", k = 2
Output
2
Every window of size 2 contains 2 vowels at most.