32. Longest Valid Parentheses

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 "()()"

class Solution(object):
    def longestValidParentheses(self, s):
        """
        :type s: str
        :rtype: int
        """
       
        n = len(s)
        if n == 0 or n == 1:
            return 0
        dp = [0] * n
        # dp[i]存储以i为结尾的最长有效匹配长度
        for i in range(1,n):
            if ")" == s[i]:
                pre = i - dp[i-1] -1
                if pre >=0 and s[pre] == "(":
                    dp[i] = dp[i-1] + 2
                    if pre > 0:
                        dp[i] += dp[pre-1]
        
        return max(dp)