Given a string containing just the characters '(' and ')', return the length of the longest valid (well-formed) parentheses substring.
A valid parentheses substring is one where every opening parenthesis '(' has a corresponding closing parenthesis ')' and they are properly nested.
0 <= s.length <= 3 * 10^4s[i] is '(' or ')'s = "(()"2s = ")()())"4s = ""0