32. Longest Valid Parentheses **堆栈

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

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

answer:

class Solution {
public:
    int longestValidParentheses(string s) {
        int res = 0, start = 0;
        stack<int> 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 :