232. 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.
  

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).
  

译：使用栈来实现一个拥有以下操作的队列：

•   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.
  

注意：
你必须仅仅使用标准的栈操作，意味着存在这几个可用的操作：push到栈顶，从栈顶peek/pop，size和is empty操作

问题分析
先画图，画分解图一步步的通过入栈出栈LIFO的特性就能反推出链表FIFO的行为，例如存在两个栈A 、B，将一组元素{a, b, c, d, e, f, g, h} 进行各种链表操作，只要遵循FIFO的行为就不难理解。