Implement Stack using Queues

描述

Implement the following operations of a stack using queues.

  • push(x) — Push element x onto stack.
  • pop() — Removes the element on top of the stack.
  • top() — Get the top element.

  • empty() — Return whether the stack is empty.
    Notes:

  • You must use only standard operations of a queue — which means only push to back, peek/pop from front, size, and is empty operations are valid.

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

    分析

可以用两个队列，q和tmp，q存放元素，tmp用来作中转。

  • push(x)，先将x push 到tmp，然后把q中的元素全部弹出来，存入tmp，最后切换q和tmp
  • pop()，直接将q的队首元素弹出来即可
    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

  1. // Implement Stack using Queues
  2. class MyStack {
  3.     // Push element x onto stack.
  4.     // Time Complexity O(n)
  5.     public void push(int x) {
  6.         tmp.offer(x);
  7.         while (!q.isEmpty()) {
  8.             final int e = q.poll();
  9.             tmp.offer(e);
  10.         }
  11.         // swap q and tmp
  12.         Queue temp = tmp;
  13.         tmp = q;
  14.         q = temp;
  15.     }
  17.     // Removes the element on top of the stack.
  18.     // Time Complexity O(1)
  19.     public void pop() {
  20.         q.poll();
  21.     }
  23.     // Get the top element.
  24.     public int top() {
  25.         return q.peek();
  26.     }
  28.     // Return whether the stack is empty.
  29.     public boolean empty() {
  30.         return q.isEmpty();
  31.     }
  33.     private Queue q = new LinkedList<>();
  34.     private Queue tmp = new LinkedList<>();
  35. }

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