Maximum Number of Vowels in a Substring of Given Length
<p>Given a string <code>s</code> and an integer <code>k</code>, return <em>the maximum number of vowel letters in any substring of </em><code>s</code><em> with length </em><code>k</code>.</p>

<p><strong>Vowel letters</strong> in English are <code>&#39;a&#39;</code>, <code>&#39;e&#39;</code>, <code>&#39;i&#39;</code>, <code>&#39;o&#39;</code>, and <code>&#39;u&#39;</code>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;abciiidef&quot;, k = 3
<strong>Output:</strong> 3
<strong>Explanation:</strong> The substring &quot;iii&quot; contains 3 vowel letters.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;aeiou&quot;, k = 2
<strong>Output:</strong> 2
<strong>Explanation:</strong> Any substring of length 2 contains 2 vowels.
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;leetcode&quot;, k = 3
<strong>Output:</strong> 2
<strong>Explanation:</strong> &quot;lee&quot;, &quot;eet&quot; and &quot;ode&quot; contain 2 vowels.
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= s.length &lt;= 10<sup>5</sup></code></li>
	<li><code>s</code> consists of lowercase English letters.</li>
	<li><code>1 &lt;= k &lt;= s.length</code></li>
</ul>
String; Sliding Window
class Solution {
public:
    int maxVowels(string s, int k) {
        auto isVowel = [](char c) {
            return c == 'a' || c == 'e' || c == 'i' || c == 'o' || c == 'u';
        };
        int cnt = count_if(s.begin(), s.begin() + k, isVowel);
        int ans = cnt;
        for (int i = k; i < s.size(); ++i) {
            cnt += isVowel(s[i]) - isVowel(s[i - k]);
            ans = max(ans, cnt);
        }
        return ans;
    }
};