32\. Longest Valid Parentheses

Hard

Given a string containing just the characters `'('` and `')'`, find the length of the longest valid (well-formed) parentheses substring.

**Example 1:**

**Input:** s = "(()"

**Output:** 2

**Explanation:** The longest valid parentheses substring is "()". 

**Example 2:**

**Input:** s = ")()())"

**Output:** 4

**Explanation:** The longest valid parentheses substring is "()()". 

**Example 3:**

**Input:** s = ""

**Output:** 0 

**Constraints:**

*   <code>0 <= s.length <= 3 * 10<sup>4</sup></code>
*   `s[i]` is `'('`, or `')'`.