description:

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

找符合规则的最长的括号

Note:

Example:

Example 1:Input: "(()"Output: 2Explanation: The longest valid parentheses substring is "()"Example 2:Input: ")()())"Output: 4Explanation: The longest valid parentheses substring is "()()"

answer:

class Solution {public:    int longestValidParentheses(string s) {        int res = 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();                    res = m.empty() ? max(res, i - start + 1) : max(res, i - m.top());                    //取出去之后同样有两种情况讨论                }            }        }        return res;    }};
    

relative point get√：

hint :