Added README.md file for Find the Longest Substring Containing Vowels in Even Counts

Author: mrprashantkumar

1473-find-the-longest-substring-containing-vowels-in-even-counts/README.md

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+<h2><a href="https://leetcode.com/problems/find-the-longest-substring-containing-vowels-in-even-counts">Find the Longest Substring Containing Vowels in Even Counts</a></h2> <img src='https://img.shields.io/badge/Difficulty-Medium-orange' alt='Difficulty: Medium' /><hr><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>