1021. 删除最外层的括号

class Solution:
    def removeOuterParentheses(self, s: str) -> str:
        ret, stack = [], []
        for i in s:
            if i == '(':
                stack.append(i)
                if len(stack) > 1:
                    ret.append(i)
            else:
                stack.pop()
                if len(stack) > 0:
                    ret.append(i)

        return ''.join(ret)