leetcode 921. 使括号有效的最少添加（Python）

 1 class Solution:
 2     def minAddToMakeValid(self, S):
 3         """
 4         :type S: str
 5         :rtype: int
 6         """
 7         stack = []
 8         S = list(S)
 9         for s in S:
10             if len(stack) != 0:
11                 if s == ")" and stack[-1] == "(":
12                     stack.pop()
13                 else:
14                     stack.append(s)
15             else:
16                 stack.append(s)
17         return len(stack)