921 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 ')'.

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12

class Solution:
    def minAddToMakeValid(self, s: str) -> int:
        count = 0
        stack = 0
        for c in s:
            if c == '(':
                stack += 1
            elif stack:
                stack -= 1
            else:
                count += 1
        return count + stack