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

2026, Jan 20 2 mins read

1. Description

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’.

2. Example

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.

3. Constraints

1 <= s.length <= 10$^{5}$
s consists of lowercase English letters.
1 <= k <= s.length

4. Solutions

Sliding Window

n = s.size()
Time complexity: O(n)
Space complexity: O(1)

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class Solution {
public:
    int maxVowels(const string &s, int k) {
        int current_vowel = 0, max_vowel = 0;
        for (int i = 0; i < k; ++i) {
            current_vowel += count_vowel(s[i]);
        }

        max_vowel = current_vowel;
        for (int i = k; i < s.size(); ++i) {
            current_vowel = current_vowel + count_vowel(s[i]) - count_vowel(s[i - k]);
            max_vowel = max(current_vowel, max_vowel);
        }

        return max_vowel;
    }

private:
    inline int count_vowel(char c) {
        switch (c) {
        case 'a':
        case 'e':
        case 'i':
        case 'o':
        case 'u':
            return 1;
        default:
            return 0;
        }
    }
};

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