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.

       大意是寻找字符串里面 "()"匹配最长的长度。最直接的思路是遍历字符串，如果是 '('，则判断下一个字符是否是 ')'，如果是，则继续向下判断，同时记录当前最长的字符长度。思路比较直接。时间复杂度：O(n)。

int longestValidParentheses(string const& s)//Longest Valid Parentheses  {      int i=0;      int length=s.length();//字符串长度      int long_length=0,max=0;      while(i