用两个栈实现队列

# -*- coding:utf-8 -*-
class Solution:
    # 初始化栈为空列表
    def __init__(self):
        self.acceptStack=[]
        self.outputStack=[]        
    def push(self, node):
        self.acceptStack.append(node)
        # write code here
      #栈是先进后出，队列是先进先出
    def pop(self):
        if self.outputStack==[]:
            while self.acceptStack!=[]:
                self.outputStack.append(self.acceptStack.pop())
        if self.outputStack !=[]:
            return self.outputStack.pop()
        else:
            return None
                
        
        # return xx