1. 注意这道题没有给初始化函数，要自己初始化两个栈
  2. 后面使用的时候记得加self. 
class Solution:
    def __init__(self):
        self.stack1 = []
        self.stack2 = []
    def push(self, node):
        # write code here
        self.stack1.append(node)
    def pop(self):
        # return xx
        if not self.stack1 and not self.stack2:
            return None
        if self.stack2 == []:
            while self.stack1:
                self.stack2.append(self.stack1.pop())
        return self.stack2.pop()