Longest Valid Parentheses (Java)

Leetcode

Longest Valid Parentheses 

Solution:
Use stack to record  '('  position, then check current valid length when a ')' come.
then, max length valid Parentheses is decided by two situation
1) stack is not empty, so the current length is current position i- last second position of '(' in stack, we can calculate it
through stack.pop(), then i-stack.peek() and check the length with max
2) stack is empty, then the longest length we can check currently is i-last (last is the position of last invalid ')')