LC.921. Minimum Add to Make Parentheses Valid

class Solution:
    """
    括号匹配 
    看剩下多少了
    """
    def minAddToMakeValid(self, S: str) -> int:
        stack = []
        for char in S:
            if char == "(":
                stack.append(char)
            else:
                if len(stack) and stack[-1] == "(":
                    stack.pop()
                else:
                    stack.append(char)

        return len(stack)