Given a string containing only the characters '(' and ')', return the length of the longest valid (well-formed) parentheses substring. A valid parentheses substring has every opening parenthesis matched by a closing parenthesis in the correct order.
Examples:
Input: s = '(()'
Output: 2
Explanation: The longest valid parentheses substring is '()'
Input: s = '()()'
Output: 4
Explanation: The entire string is a valid parentheses substring
Input: s = '(())'
Output: 4
Explanation: The entire string is a valid parentheses substring
Input: s = ')'
Output: 0
Explanation: No valid parentheses substring exists
Input: s = ''
Output: 0
Explanation: Empty string has no valid parentheses substring
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