Added README.md file for Longest Valid Parentheses

Author: joshua-dierickse-alt

32-longest-valid-parentheses/README.md

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+<h2><a href="https://leetcode.com/problems/longest-valid-parentheses">Longest Valid Parentheses</a></h2> <img src='https://img.shields.io/badge/Difficulty-Hard-red' alt='Difficulty: Hard' /><hr><p>Given a string containing just the characters <code>&#39;(&#39;</code> and <code>&#39;)&#39;</code>, return <em>the length of the longest valid (well-formed) parentheses </em><span data-keyword="substring-nonempty"><em>substring</em></span>.</p>
+
+<p>&nbsp;</p>
+<p><strong class="example">Example 1:</strong></p>
+
+<pre>
+<strong>Input:</strong> s = &quot;(()&quot;
+<strong>Output:</strong> 2
+<strong>Explanation:</strong> The longest valid parentheses substring is &quot;()&quot;.
+</pre>
+
+<p><strong class="example">Example 2:</strong></p>
+
+<pre>
+<strong>Input:</strong> s = &quot;)()())&quot;
+<strong>Output:</strong> 4
+<strong>Explanation:</strong> The longest valid parentheses substring is &quot;()()&quot;.
+</pre>
+
+<p><strong class="example">Example 3:</strong></p>
+
+<pre>
+<strong>Input:</strong> s = &quot;&quot;
+<strong>Output:</strong> 0
+</pre>
+
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+	<li><code>0 &lt;= s.length &lt;= 3 * 10<sup>4</sup></code></li>
+	<li><code>s[i]</code> is <code>&#39;(&#39;</code>, or <code>&#39;)&#39;</code>.</li>
+</ul>