leetcode 32 最长有效括号

class (object):
    def longestValidParentheses(self, s):
        """
        :type s: str
        :rtype: int
        """
        stack = [0]
        lmax = 0
        if not s:
            return 0
        for i in s:
            if i == '(':
                stack.append(0)
            else:
                if len(stack) <=1:
                    stack = [0]
                else:
                    stack[-2] += stack[-1]+2
                    stack.pop()
                    lmax = max(lmax, stack[-1])
        return lmax