[LeetCode] CMaximum Number of Vowels in a Substring of Given Length

題目：Maximum Number of Vowels in a Substring of Given Length
難度：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', 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

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解題技巧為 Sliding Window，跟 Maximum Average Subarray I 一樣，差別只在於這題要做母音的判斷

public class Solution {
    public int MaxVowels(string s, int k) {
        int currentCount = 0;
        int maxCount;

        for (int i = 0; i < k; i ++)
        {
            if (IsVowel(s[i]))
            {
                currentCount++;
            }
        }

        maxCount = currentCount;

        for (int j = 0; j+k < s.Length; j++)
        {
            if (IsVowel(s[j]))
            {
                currentCount--;
            }

            if (IsVowel(s[j+k]))
            {
                currentCount++;
            }

            maxCount = Math.Max(maxCount,currentCount);
        }

        return maxCount;
    }

    /// <Summary>
    /// 該字母是否為母音
    /// </Summary>    
    public bool IsVowel(char c)
    {
        if (c == 'a' ||c == 'e' ||c == 'i' ||c == 'o' ||c == 'u')
        {
            return true;
        }

        return false;
    }
}