Leetcode - Minimum Add to Make Parentheses Valid Solution

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.

Solution in Python

class Solution:
    def minAddToMakeValid(self, S: str) -> int:
        stack = []
        for i in S:
            if i == "(" or not stack:
                stack.append(i)
            elif i == ")" and stack[-1]== "(":
                stack.pop()
            else:
                stack.append(i)
        return len(stack)