Leetcode - Minimum Add to Make Parentheses Valid Solution

# 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)