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
        """
        maxlen = 0
        stack = list()
        last = -1
        for i in range(len(s)):
            if s[i] == '(':
                stack.append(i)
            else:
                if stack == []:
                    last = i
                else:
                    stack.pop()
                    if stack == []:
                        maxlen = max(maxlen, i- last)
                    else:
                        maxlen = max(maxlen, i-stack[len(stack)-1])
        return maxlen

一切还可以简化：

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