题目
Given a string containing just the characters ‘(‘ and ‘)’, find the length of the longest valid (well-formed) parentheses substring.
解题思路
使用栈，栈存坐标，栈底元素为最后一个没被匹配的右括号，遇到左括号进栈，遇到右括号出栈，并更新结果（如果栈为空，则这个是最后一个没被匹配的右括号）
代码
class Solution:
def longestValidParentheses(self, s: str) -> int:
stack = [-1]
res = 0
for i, c in enumerate(s):
if c == '(':
stack.append(i)
else:
stack.pop()
if not stack:
stack.append(i)
else:
res = max(res, i - stack[-1])
return res
