Implement Queue using Stacks

描述

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).

    分析

可以用两个栈，s和tmp，s存放元素，tmp用来作中转。

  • push(x)，先将s中的元素全部弹出来，存入tmp，把x push 到tmp，然后把tmp中的元素全部弹出来，存入s
  • pop()，直接将s的栈顶元素弹出来即可
    该算法push的算法复杂度是O(n), pop的算法复杂度O(1)。

另个一个方法是，让pop是O(n), push是O(1)，思路很类似，就不赘述了。

代码

相关题目

原文: https://soulmachine.gitbooks.io/algorithm-essentials/content/cpp/stack-and-queue/queue/implement-queue-using-stacks.html