题目：
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.
链接： http://leetcode.com/problems/longest-valid-parentheses/
题解：
测试：
原文：http://www.cnblogs.com/yrbbest/p/4435218.html