Minimum Add to Make Parentheses Valid

Problem Statement

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.

Leetcode link

Example 1:

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

Example 2:

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

Constraints:

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

Code

Python Code

class Solution:
    def minAddToMakeValid(self, S):
        """
        :type S: str
        :rtype: int
        """
        cnt,cnt2 = 0, 0
        for i in range(len(S)):
            if S[i] == '(':
                cnt += 1
            elif cnt > 0 and S[i] == ')':
                cnt -= 1
        for i in range(len(S) - 1, -1, -1):
            if S[i] == ')':
                cnt2 += 1
            elif cnt2 > 0 and S[i] == '(':
                cnt2 -= 1
        return cnt + cnt2