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.
https://oj.leetcode.com/problems/longest-valid-parentheses/
分析，求出一串由：‘(’和‘)’组成的字符串中合法()配对的长度。例如：(()()，结果是4。((()))结果是6。())()()结果是4。
解题思路：最容易想到的解法就是穷举法，计算任意两点(i到j)之间有多少个合法的()串，借助动态规划可以得到结果。算法复杂度为：
想要O(n)的解法需要一点技巧，也要弄清楚几种情况：
AC代码如下所示：
public class Solution {
public int longestValidParentheses(String s) {
if(s==null||s.length()==0) {
return 0;
}
int start = -1;
int maxLength = 0;
Stack stack = new Stack();
for(int i=0;i
|