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 “()()”

翻譯
給定一個只包含 ‘(’ 和 ‘)’ 的字串，找出最長的包含有效括號的子串的長度。

示例 1:
輸入: "(()"
輸出: 2
解釋: 最長有效括號子串為 “()”

示例 2:
輸入: ")()())"
輸出: 4
解釋: 最長有效括號子串為 “()()”

分析
很久之前寫的解法了，複雜度為O(n^2)，由於資料量不大勉強過了。正確做法還是應該用棧或者動態規劃。

c++實現

class Solution {
public:
    int longestValidParentheses(string s) {
        int res = 0;
        bool found =
 
 false;
        for (int i = 0; i < s.length(); i++)
        {
            int length = 0;
            int tmp = 0;
            int start = i;
            for (int j = i; j < s.length(); j++)
            {
                if (s[j] == '(')
                    tmp++;
                else
                    tmp--
 
;
                if (tmp == 0)
                {
                    length += j+1-start;
                    start = j+1;
                    if (res < length)
                        res = length;
                }
                if (tmp < 0)
                {
                    length = 0;
                    start = j+1;
                    tmp = 0;
                }
            }
        }
        return res;
    }
};