32. 最长有效括号

问题描述

给定一个只包含 '(' 和 ')' 的字符串，找出最长的包含有效括号的子串的长度。

方法1：栈

class Solution:
    def longestValidParentheses(self, s: str) -> int:
        # 栈
        lenth = len(s)
        stack = [-1]
        res = 0
        for i in range(lenth):
            if s[i] == '(':
                stack.append(i)
            else:
                stack.pop()
                if not stack:
                    stack.append(i)
                else:
                    res = max(res, i - stack[-1])     

        return res

方法2：贪心一点

class Solution:
    def longestValidParentheses(self, s: str) -> int:
        #贪心一点，左-->右，右-->左
        lenth = len(s)
        res = 0
        # 左--> 右 
        left = right = 0
        for val in s:
            if val == '(':
                left += 1
            else:
                right += 1
            if left == right:
                res = max(res, 2 * left)
            elif right > left:
                left = right = 0

        # 右-->左
        left = right = 0
        for i in range(lenth - 1, -1, -1):
            if s[i] == ')':
                right += 1
            else:
                left += 1
            if left == right:
                res = max(res, 2 * left)
            elif left > right:
                left = right = 0
        
        return res