Minimum Add to Make Parentheses Valid

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:
class Solution:
    def minAddToMakeValid(self, s: str) -> int:
        stack = []
        ctr = 0
        for c in s:
            if c == '(':
                stack.append('(')
            else:
                if len(stack) == 0 or stack[-1] != '(':
                    ctr += 1
                else:
                    stack.pop()
        ctr += len(stack)
        return ctr

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