[LeetCode] 921. Minimum Add to Make Parentheses Valid

 Problem : https://leetcode.com/problems/minimum-add-to-make-parentheses-valid/

Use stack to match left parenthesis with right parenthesis as much as possible.

class Solution:
    def minAddToMakeValid(self, s: str) -> int:
        stack = []
        
        for w in s:
            if w == '(':
                stack.append(w)
            elif stack and stack[-1] == '(':
                stack.pop()
            else:
                stack.append(w)
        
        return len(stack)