Longest valid parentheses

[vbrazo]

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