<p>Given the string <code>s</code>, return the size of the longest substring containing each vowel an even number of times. That is, &#39;a&#39;, &#39;e&#39;, &#39;i&#39;, &#39;o&#39;, and &#39;u&#39; must appear an even number of times.</p>

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

<pre>
<strong>Input:</strong> s = &quot;eleetminicoworoep&quot;
<strong>Output:</strong> 13
<strong>Explanation: </strong>The longest substring is &quot;leetminicowor&quot; which contains two each of the vowels: <strong>e</strong>, <strong>i</strong> and <strong>o</strong> and zero of the vowels: <strong>a</strong> and <strong>u</strong>.
</pre>

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

<pre>
<strong>Input:</strong> s = &quot;leetcodeisgreat&quot;
<strong>Output:</strong> 5
<strong>Explanation:</strong> The longest substring is &quot;leetc&quot; which contains two e&#39;s.
</pre>

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

<pre>
<strong>Input:</strong> s = &quot;bcbcbc&quot;
<strong>Output:</strong> 6
<strong>Explanation:</strong> In this case, the given string &quot;bcbcbc&quot; is the longest because all vowels: <strong>a</strong>, <strong>e</strong>, <strong>i</strong>, <strong>o</strong> and <strong>u</strong> appear zero times.
</pre>

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

<ul>
	<li><code>1 &lt;= s.length &lt;= 5 x 10^5</code></li>
	<li><code>s</code>&nbsp;contains only lowercase English letters.</li>
</ul>

class Solution {
  public:
    int findTheLongestSubstring(std::string s) {
        std::vector<char> vowels = {'a', 'e', 'i', 'o', 'u'};

        for (int window = s.size(); window > 0; window--) {
            std::unordered_map<char, int> count;
            for (int i = 0; i < window; i++) {
                count[s[i]]++;
            }

            if (isValid(vowels, count)) {
                return window;
            }

            int start = 0, end = window - 1;

            while (end < s.size() - 1) {
                count[s[start]]--;
                if (count[s[start]] < 0) {
                    count[s[start]] = 0;
                }
                start++, end++;
                count[s[end]]++;
                if (isValid(vowels, count)) {
                    return window;
                }
            }
        }

        return 0;
    }

  private:
    bool isValid(std::vector<char>             &vowels,
                 std::unordered_map<char, int> &count) {
        for (char v : vowels) {
            if (count[v] % 2 != 0) {
                return false;
            }
        }
        return true;
    }
};
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