<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>

#include <bits/stdc++.h>
using namespace std;

class Solution {
  public:
    static void updateMask(char& mask, char c) {
        switch (c) {
            case 'a':
                mask ^= (1 << 4);
                break;
            case 'e':
                mask ^= (1 << 3);
                break;
            case 'i':
                mask ^= (1 << 2);
                break;
            case 'o':
                mask ^= (1 << 1);
                break;
            case 'u':
                mask ^= (1 << 0);
                break;
        }
    }

    static int findTheLongestSubstring(string s) {
        std::vector<char> masks(s.size(), 0);
        updateMask(masks[0], s[0]);
        int max = 0;
        if (masks[0] == 0) {
            max = 1;
        }
        for (size_t idx = 1; idx < s.size(); ++idx) {
            masks[idx] = masks[idx - 1];
            updateMask(masks[idx], s[idx]);
            if (masks[idx] == 0) {
                max = idx + 1;
            }
        }
        for (size_t idx = 1; idx < masks.size(); ++idx) {
            for (size_t bidx = 0; bidx < idx; ++bidx) {
                if (static_cast<int>(idx - bidx) < max) {
                    break;
                }
                if ((masks[idx] ^ masks[bidx]) == 0) {
                    max = idx - bidx;
                }
            }
        }
        return max;
    }
};static const auto kds = []() {
    std::ios::sync_with_stdio(false);
    std::cin.tie(nullptr);
    std::cout.tie(nullptr);
    return 0;
}();
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