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 "()()"

AC code:

class Solution {public:    int longestValidParentheses(string s) {        int n = s.length(), longest = 0;        stack
     
       st;        for (int i = 0; i < n; i++) {            if (s[i] == '(') st.push(i);            else {                if (!st.empty()) {                    if (s[st.top()] == '(') st.pop();                    else st.push(i);                }                else st.push(i);            }        }        if (st.empty()) longest = n;        else {            int a = n, b = 0;            while (!st.empty()) {                b = st.top(); st.pop();                longest = max(longest, a-b-1);                a = b;            }            longest = max(longest, a);        }        return longest;    }};
     

Runtime: 

8 ms, faster than 62.98% of C++ online submissions for Longest Valid Parentheses.