Leetcode每日一题——T32. 最长有效括号（难）——栈

法一：栈

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