Coursera Algorithms week2 栈和队列练习测验: Queue with two stacks

最新推荐文章于 2018-05-22 14:10:54 发布

转载最新推荐文章于 2018-05-22 14:10:54 发布 · 186 阅读

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原文链接：http://www.cnblogs.com/evasean/p/7220077.html

本文介绍了一种使用两个栈来实现队列的方法，确保每个队列操作都能在常数级的摊销时间内完成。通过巧妙地在两个栈之间转移元素，实现了队列的基本功能：入队、出队等。

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题目原文：

Implement a queue with two stacks so that each queue operations takes a constant amortized number of stack operations.

题目要求用栈实现队列的所有操作。

 1 package week2;
 2 
 3 import java.util.Stack;
 4 
 5 /**
 6  * Queue with two stacks. Implement a queue with two stacks so that each queue 
 7  * operations takes a constant amortized number of stack operations
 8  * @author yangjingjing
 9  *
10  */
11 public class QueueWith2Stacks<E>{
12     Stack<E> inStack = new Stack<E>();
13     Stack<E> outStack = new Stack<E>();
14     public boolean isEmpty(){
15         if(inStack.isEmpty() && outStack.isEmpty())
16             return true;
17         else return false;
18     }
19     public int size(){
20         return (inStack.size() + outStack.size());
21     }
22     public void enqueue(E item){
23         inStack.push(item);
24     }
25     public E dequeue(){
26         if(outStack.isEmpty()){
27             if(inStack.isEmpty()) return null;
28             else{
29                 while(!inStack.isEmpty()) {
30                     outStack.push(inStack.pop());
31                 }
32                 return outStack.pop();
33             }
34         }else{
35             return outStack.pop();
36         }
37     }
38     public static void main(String[] args) {
39         QueueWith2Stacks<Object> queue = new QueueWith2Stacks<Object>();
40         queue.enqueue("a");
41         queue.enqueue("b");
42         queue.enqueue("c");
43         System.out.println(queue.dequeue());
44         queue.enqueue(1);
45         queue.enqueue(2);
46         Object o = null;
47         while((o = queue.dequeue()) != null) {
48             System.out.println(o);
49         }
50     }
51 }