# -*- coding:utf-8 -*-

class Solution:

    def __init__(self):

        self.stackA= []

        self.stackB= []

    def push(self, node):

        # write code here

        self.stackA.append(node)

    def pop(self):

        # return xx

        if self.stackB:

            return  self.stackB.pop()

        elif not self.stackA:

            return None

        else:

            while self.stackA:

                self.stackB.append(self.stackA.pop())

        return self.stackB.pop()