Longest Valid Parentheses

Hard

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

Example 1

Input:  s = "(()"
Output: 2

Example 2

Input:  s = ")()())"
Output: 4

Example 3

Input:  s = ""
Output: 0

Constraints

• 0 <= s.length <= 16
• s[i] is '(' or ')'.