leetcode 232 Implement Queue usi

原题链接：https://leetcode.com/problems/implement-queue-using-stacks/

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.
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).
每次入栈只需要往enstack中入栈，出栈或查看栈顶值时，先判断destack中是否有值， 若有则可直接出栈，因为在enstack中的值肯定比destack中晚入栈。 若没有，则把enstack值全部入栈至destack中，再让destack栈顶值出栈。

class MyQueue:

    def __init__(self):
        """
        Initialize your data structure here.
        """
        self.enstack = []
        self.destack = []

    def push(self, x: int) -> None:
        """
        Push element x to the back of queue.
        """
        self.enstack.append(x)
        

    def pop(self) -> int:
        """
        Removes the element from in front of queue and returns that element.
        """
        while self.destack:
            return self.destack.pop()
        while self.enstack:
            self.destack.append(self.enstack.pop())
        return self.destack.pop()

    def peek(self) -> int:
        """
        Get the front element.
        """
        while self.destack:
            return self.destack[-1]
        while self.enstack:
            self.destack.append(self.enstack.pop())
        return self.destack[-1]
        

    def empty(self) -> bool:
        """
        Returns whether the queue is empty.
        """
        return not (self.enstack or self.destack)