算法描述：

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

Example 1:

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

Example 2:

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

解題思路：用棧模擬，遇到左括號進棧，右括號出棧，並計算合法括號數量。

    int longestValidParentheses(string s) {
        stack<int> stk;
        int res = 0;
        int i = 0;
        int start = -1;
        while(i < s.size()){
            if(s[i]==‘(‘){
                stk.push(i);
            }else{
                if(stk.empty())
                    start 
 
= i;
                else{
                    stk.pop();
                    if(stk.empty())
                        res = max(res, i- start);
                    else
                        res = max(res, i-stk.top());
                }
            }
            i++;
        }
        return res;
    }
 

LeetCode-32-Longest Valid Parentheses