[Leetcode] 32. Longest Valid Parentheses

Given a string containing just the characters '(' and ')', find the length of the longest valid (well-formed) parentheses substring.

For "(()", the longest valid parentheses substring is "()", which has length = 2.

Another example is ")()())", where the longest valid parentheses substring is "()()", which has length = 4.

寻找最长合法括号字串

方法：

建立一个栈，初始时里面只有-1。

碰到 "(" 直接入栈。

碰到 “)” ，先弹栈，如果栈空，则将该括号的index入栈。否则，用该index 减去 栈顶元素（也就是这个括号最远能匹配到的左括号），得到当前最长合法字串的长度。

通过上述规则，记录得到全局的最长即可。

class Solution(object):    def longestValidParentheses(self, s):        """        :type s: str        :rtype: int        """        stack = [-1]        maxx = 0        for i in range(0,len(s)):            if s[i] == '(':                stack.append(i)            else:                stack.pop()                if len(stack) == 0:                    stack.append(i)                else:                    top = len(stack)-1                    maxx = max(i-stack[top],maxx)        return maxx