921. Minimum Add to Make Parentheses Valid

Problem Link

Description

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A parentheses string is valid if and only if:

• It is the empty string,
• 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.

You are given a parentheses string s. In one move, you can insert a parenthesis at any position of the string.

• For example, if s = "()))", you can insert an opening parenthesis to be "(()))" or a closing parenthesis to be "())))".

Return the minimum number of moves required to make s valid.

 

Example 1:

Input: s = "())"
Output: 1

Example 2:

Input: s = "((("
Output: 3

 

Constraints:

• 1 <= s.length <= 1000
• s[i] is either '(' or ')'.

Solution

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Python3

class Solution:
    def minAddToMakeValid(self, s: str) -> int:
        needLeft, needRight = 0, 0
        
        for i in range(len(s)):
            if s[i] == '(':
                needRight += 1
            elif s[i] == ')':
                if needRight:
                    needRight -= 1
                else:
                    needLeft += 1
        
        return needLeft + needRight