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 "()()"

方法1：

class Solution {   
    public int longestValidParentheses(String s) {
        int maxlen = 0;
        for (int i = 0; i < s.length(); i++) {
            for (int j = i + 2; j <= s.length(); j+=2) {
                if (isValid(s.substring(i, j))) {
                    maxlen = Math.max(maxlen, j - i);
                }
            }
        }
        return maxlen;
    }
        
         
    public boolean isValid(String s) {
        Stack<Character> stack = new Stack<Character>();
        for (int i = 0; i < s.length(); i++) {
            if (s.charAt(i) == '(') {
                stack.push('(');
            } else if (!stack.empty() && stack.peek() == '(') {
                stack.pop();
            } else {
                return false;
            }
        }
        return stack.empty();
    }
}
 

時間複雜度：O(n^2)

空間複雜度：O(n)