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 either '(' or ')'