9: Use two stacks to implement queues (expansion: use two queues to implement one stack)

Using two stacks to implement a queue, the statement is as follows: To implement Push and Pop operations of the queue, that is, to insert a node at the end of the queue and delete the inserted node at the head of the queue. The elements in the queue are of type int.

class Queue:
    def __init__(self):
        self.stack1 = []
        self.stack2 = []
        # self.stack1 = [x for x in list]
        # self.stack2 = []这种方式错误，并不是一开始就给出一个队列，而是自己组建一个队列并实现其功能

    def push(self, node):
        return self.stack1.append(node)
        # 因为是队列所以不能用切片操作，因为体现不出过程
        # self.stack1.append(x)
        # self.stack2 = self.stack1[::-1]

    def pop(self):
        if self.stack2:
            return self.stack2.pop()
        elif not self.stack1:
            return None
        else:
            self.stack2.append(self.stack1.pop())
            return self.stack2.pop()
        # self.stack1.pop()
        # self.stack2 = self.stack1[::-1]
        # return self.stack2

Topic expansion: using two queues to implement a stack

class Solution:
    def __init__(self):
        self.queue1 = []
        self.queue2 = []

    def push(self, node):
        return self.queue1.append(node)

    def pop(self):
        if not self.queue1:
            return None
        else:
            while len(self.queue1)!= 1:
                self.queue2 =self.queue1.pop()
            self.queue1,self.queue2 =self.queue2,self.queue1
            #这一步交换不能少，否则就只能执行一次pop操作
            return self.queue2.pop()
# pycharm alt+j是选中多个单词