# -*- 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()