Given a string containing just the characters '(' and ')', find the length of the longest valid (well-formed) parentheses substring.

Example 1:

Input: "(()"Output: 2Explanation: The longest valid parentheses substring is "()"

Example 2:

Input: ")()())"Output: 4Explanation: The longest valid parentheses substring is "()()"

代码：

class Solution {public:    int longestValidParentheses(string s) {        int ans = 0, start = 0;        stack
     
       m;        for (int i = 0; i < s.size(); ++i) {            if (s[i] == '(') m.push(i);            else if (s[i] == ')') {                if (m.empty()) start = i + 1;                else {                    m.pop();                    if(m.empty())                         ans = max(ans, i - start + 1);                    else ans = max(ans, i - m.top());                }            }        }        return ans;    }};
     

　　最长有效括号