921. 使括号有效的最少添加

任务描述

思路

• 栈

代码实现

class Solution:
    def minAddToMakeValid(self, S: str) -> int:
        if not S:
            return 0
        stack = []
        stack.append(S[0])
        c1 = 1
        for i in S[1:]:
            if len(stack)!=0 and stack[-1]+i=='()':
                c1 -= 1
                stack.pop()
            else:
                c1 += 1
                stack.append(i)
        return c1