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
0 <= s.length <= 3 * 104 s[i] is '(', or ')'.