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:

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