待补充。
Given a string containing just the characters ‘(‘ and ‘)‘, find the length of the longest valid (well-formed) parentheses substring.
For "(()", the longest valid parentheses substring is "()", which has length = 2.
Another example is ")()())", where the longest valid parentheses substring is "()()", which has length = 4.
[LeetCode]39. Longest Valid Parentheses最长有效括号对
原文：http://www.cnblogs.com/aprilcheny/p/4907320.html