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

  1. // Implement Queue using Stacks
  2. class MyQueue {
  3.     // Push element x to the back of queue.
  4.     // Time Complexity: O(n)
  5.     public void push(int x) {
  6.         while (!s.isEmpty()) {
  7.             final int e = s.pop();
  8.             tmp.push(e);
  9.         }
  10.         tmp.push(x);
  12.         while(!tmp.isEmpty()) {
  13.             final int e = tmp.pop();
  14.             s.push(e);
  15.         }
  16.     }
  18.     // Removes the element from in front of queue.
  19.     // Time Complexity: O(1)
  20.     public void pop() {
  21.         s.pop();
  22.     }
  24.     // Get the front element.
  25.     public int peek() {
  26.         return s.peek();
  27.     }
  29.     // Return whether the queue is empty.
  30.     public boolean empty() {
  31.         return s.isEmpty();
  32.     }
  34.     private final Stack s = new Stack<>();
  35.     private final Stack tmp = new Stack<>();
  36. }

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