<h2><a href="https://leetcode.com/problems/longest-valid-parentheses/">32. Longest Valid Parentheses</a></h2><h3>Hard</h3><hr><div><p>Given a string containing just the characters <code>'('</code> and <code>')'</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 = "(()"
<strong>Output:</strong> 2
<strong>Explanation:</strong> The longest valid parentheses substring is "()".
</pre>

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

<pre><strong>Input:</strong> s = ")()())"
<strong>Output:</strong> 4
<strong>Explanation:</strong> The longest valid parentheses substring is "()()".
</pre>

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

<pre><strong>Input:</strong> s = ""
<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>'('</code>, or <code>')'</code>.</li>
</ul>
</div>