## 1. Problem

1456. Maximum Number of Vowels in a Substring of Given Length

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

Vowel letters in English are ‘a’, ‘e’, ‘i’, ‘o’, and ‘u’.

**Example 1:**

```
Input: s = "abciiidef", k = 3
Output: 3
Explanation: The substring "iii" contains 3 vowel letters.
```

**Example 2:**

```
Input: s = "aeiou", k = 2
Output: 2
Explanation: Any substring of length 2 contains 2 vowels.
```

**Example 3:**

```
Input: s = "leetcode", k = 3
Output: 2
Explanation: "lee", "eet" and "ode" contain 2 vowels.
```

**Constraints:**

- 1 <= s.length <= 10^5
- s consists of lowercase English letters.
- 1 <= k <= s.length

## 2. Solution

I solved this problem like this.

```
class Solution:
def maxVowels(self, s: str, k: int) -> int:
vowels = ['a', 'e', 'i', 'o', 'u']
v_cnt = sum([1 if x in vowels else 0 for x in s[:k]])
answer = v_cnt
for i in range(k, len(s)):
if s[i] in vowels:
v_cnt += 1
if s[i-k] in vowels:
v_cnt -= 1
answer = max(answer, v_cnt)
return answer
```