921. Minimum Add to Make Parentheses Valid

Given a string S of '(' and ')' parentheses, we add the minimum number of parentheses ( '(' or ')', and in any positions ) so that the resulting parentheses string is valid.

Formally, a parentheses string is valid if and only if:

• It is the empty string, or
• It can be written as AB (A concatenated with B), where A and B are valid strings, or
• It can be written as (A), where A is a valid string.

Given a parentheses string, return the minimum number of parentheses we must add to make the resulting string valid.

Example 1:

Input: "())"
Output: 1

Example 2:

Input: "((("
Output: 3

Example 3:

Input: "()"
Output: 0

Example 4:

Input: "()))(("
Output: 4

Note:

1. S.length <= 1000
2. S only consists of '(' and ')' characters.

class Solution:
    def minAddToMakeValid(self, S):
        """
        :type S: str
        :rtype: int
        """
        res = 0
        count = 0
        for i in S:
            if i=='(':
                count += 1
            else:
                if count:
                    count -= 1
                else:
                    res += 1
        return res+count

一开始看到括号匹配下意识地用了栈，但仔细一想好像没必要用栈。顺便吐槽这其实应该是一道easy的题目。