This is a straightforward fixed-size sliding window problem. We maintain a count of vowels in the current window of size k.

1. Vowels set: {'a', 'e', 'i', 'o', 'u'}.
2. Calculate vowels in the first k characters.
3. res = count.
4. Slide the window:
   • If incoming char is vowel, count += 1.
   • If outgoing char is vowel, count -= 1.
   • res = max(res, count).
5. Return res.

• Time Complexity: O(N).
• Space Complexity: O(1).

def max_vowels(s, k):
    vowels = {'a', 'e', 'i', 'o', 'u'}
    count = 0
    for i in range(k):
        if s[i] in vowels:
            count += 1
            
    res = count
    for i in range(k, len(s)):
        if s[i] in vowels:
            count += 1
        if s[i - k] in vowels:
            count -= 1
        res = max(res, count)
        
    return res