Longest Valid Parentheses

Given a string containing just the characters `'('` and `')'`, return _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:** - `0 <= s.length <= 3 * 104` - `s[i]` is `'('`, or `')'`.