leetcode 921. 使括号有效的最少添加（Python）

class Solution:
    def minAddToMakeValid(self, S):
        """
        :type S: str
        :rtype: int
        """
        stack = []
        S = list(S)
        for s in S:
            if len(stack) != 0:
                if s == ")" and stack[-1] == "(":
                    stack.pop()
                else:
                    stack.append(s)
            else:
                stack.append(s)
        return len(stack)

击败了百分之百的人，纪念一下。