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.
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).
可以用两个栈来实现队列，在 pop 的时候，需要迁移栈，利用平均时间复杂度，可以算出来 pop 和 peek 都是 O(1)
注意： 这里题目要求 pop 的时候返回数据