[Pyhon3] LeetCode 232. Implement Queue using Stacks - [Easy]

232. Implement Queue using Stacks
232. 用栈实现队列
（剑指offer - 9. 用两个栈实现队列）

Implement the following operations of a queue using stacks.

• push(x) – Push element x to the back of queue.
• pop() – Removes the element from in front of queue.
• peek() – Get the front element.
• empty() – Return whether the queue is empty.

使用栈实现队列的下列操作：

• push(x) – 将一个元素放入队列的尾部。
• pop() – 从队列首部移除元素。
• peek() – 返回队列首部的元素。
• empty() – 返回队列是否为空。

Example:

MyQueue queue = new MyQueue();

queue.push(1);
queue.push(2);  
queue.peek();  // returns 1
queue.pop();   // returns 1
queue.empty(); // returns false

Notes:

• You must use only standard operations of a stack – which means only push to top, peek/pop from top, size, and is empty operations are valid.
• Depending on your language, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.
• You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).

说明:

• 你只能使用标准的栈操作 – 也就是只有 push to top, peek/pop from top, size, 和 is empty 操作是合法的。
• 你所使用的语言也许不支持栈。你可以使用 list 或者 deque（双端队列）来模拟一个栈，只要是标准的栈操作即可。
• 假设所有操作都是有效的 （例如，一个空的队列不会调用 pop 或者 peek 操作）。

class MyQueue:
    def __init__(self):
        """
        Initialize your data structure here.
        """
        self.stack_in = []
        self.stack_out = []

    def push(self, x):
        """
        Push element x to the back of queue.
        :type x: int
        :rtype: void
        """
        self.stack_in.append(x)

    def pop(self):
        """
        Removes the element from in front of queue and returns that element.
        :rtype: int
        """
        if self.stack_in == [] and self.stack_out == []:
            return None
        while self.stack_in:
            self.stack_out.append(self.stack_in.pop(0))
        return self.stack_out.pop(0)
        
    def peek(self):
        """
        Get the front element.
        :rtype: int
        """
        if self.stack_in == [] and self.stack_out == []:
            return None
        while self.stack_in:
            self.stack_out.append(self.stack_in.pop(0))
        return self.stack_out[0]
        
    def empty(self):
        """
        Returns whether the queue is empty.
        :rtype: bool
        """
        if self.stack_in == [] and self.stack_out == []:
            return True
        return False

# Your MyQueue object will be instantiated and called as such:
# obj = MyQueue()
# obj.push(x)
# param_2 = obj.pop()
# param_3 = obj.peek()
# param_4 = obj.empty()

图解参考：https://www.cnblogs.com/xugenpeng/p/10051819.html