Formatted question description: https://leetcode.ca/all/1456.html

# 1456. Maximum Number of Vowels in a Substring of Given Length (Medium)

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, 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.


Example 4:

Input: s = "rhythms", k = 4
Output: 0
Explanation: We can see that s doesn't have any vowel letters.


Example 5:

Input: s = "tryhard", k = 4
Output: 1


Constraints:

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

Related Topics:
String, Sliding Window

## Solution 1. Sliding window

We keep a sliding window of size k. Let i be the index of the new element getting into the window. We increment cnt if s[i] is vowel.

If i >= k, we need to pop s[i - k] out of the window and decrement cnt if s[i - k] is vowel. The answer is the maximum cnt.

// OJ: https://leetcode.com/problems/maximum-number-of-vowels-in-a-substring-of-given-length/

// Time: O(N)
// Space: O(1)
class Solution {
bool isVowel(char c) {
return c == 'a' || c == 'e' || c == 'i' || c == 'o' || c == 'u';
}
public:
int maxVowels(string s, int k) {
int cnt = 0, ans = 0;
for (int i = 0; i < s.size(); ++i) {
if (i >= k) cnt -= isVowel(s[i - k]);
cnt += isVowel(s[i]);
ans = max(ans, cnt);
}
return ans;
}
};


Java

class Solution {
public int maxVowels(String s, int k) {
int count = 0;
int length = s.length();
if (k > length)
return 0;
for (int i = 0; i < k; i++) {
if (isVowel(s.charAt(i)))
count++;
}
int maxCount = count;
for (int i = k; i < length; i++) {
char prevC = s.charAt(i - k), currC = s.charAt(i);
if (isVowel(prevC))
count--;
if (isVowel(currC))
count++;
maxCount = Math.max(maxCount, count);
}
return maxCount;
}

public boolean isVowel(char c) {
return c == 'a' || c == 'e' || c == 'i' || c == 'o' || c == 'u';
}
}