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。