<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:
    int findTheLongestSubstring(string s)
    {
        int n = s.size(), res = 0;
        map<vector<bool>, int> mp;   
        vector<bool> mask(5, false); 
        mp[mask] = -1;              
        unordered_map<char, int> vowel_to_bit = {{'a', 0}, {'e', 1}, {'i', 2}, {'o', 3}, {'u', 4}};

        for (int i = 0; i < n; i++)
        {
            if (vowel_to_bit.count(s[i]))
            {
                int bit_to_flip = vowel_to_bit[s[i]];   
                mask[bit_to_flip] = !mask[bit_to_flip]; 
            }
            if (mp.find(mask) == mp.end())
                mp[mask] = i; 
            else
                res = max(res, i - mp[mask]); 
        }
        return res;
    }
};

auto init = []()
{ios::sync_with_stdio(false); cin.tie(0);cout.tie(0);return 0; }();

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