普林斯顿算法课第二周作业

Programming Assignment 2: Randomized Queues and Deques

题目地址：

http://coursera.cs.princeton.edu/algs4/assignments/queues.html

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正文：

第二周作业要做两个类


第一个是使用迭代器iterator的双向队列类

public class Deque<Item> implements Iterable<Item> {   public Deque()                           // construct an empty deque   public boolean isEmpty()                 // is the deque empty?   public int size()                        // return the number of items on the deque   public void addFirst(Item item)          // insert the item at the front   public void addLast(Item item)           // insert the item at the end   public Item removeFirst()                // delete and return the item at the front   public Item removeLast()                 // delete and return the item at the end   public Iterator<Item> iterator()         // return an iterator over items in order from front to end   public static void main(String[] args)   // unit testing}

JAVA的队列与C的一致，这里采用的是线性表实现。


第一个类应该不难，无非就是两侧都可以入队、出队。
相信掌握了单队列的同学很快就可以独立编写出来。
稍微注意一下捕捉和抛出错误，按照题目里给予的方式即可，否则会扣分。
要说一下迭代器的问题。
迭代器照我的理解就是一种方便使用遍历方法的接口。
在iterator里面，我们要实现遍历的方法。
源代码如下：

import java.util.Iterator;public class Deque<Item> implements Iterable<Item> { private int N;               // number of elements on queue private Node first;    // beginning of queue private Node last;     // end of queue  private class Node { Item item; Node pre; Node next; }  public Deque(){ first = null;     last  = null;     N = 0; }                           // construct an empty deque    public boolean isEmpty(){   return N==0;   }                 // is the deque empty?    public int size(){ return N; }                        // return the number of items on the deque    public void addFirst(Item item){ if(item == null) throw new java.lang.NullPointerException(); Node oldfirst = first; first = new Node(); first.item = item; //first.pre = null; first.next=oldfirst; if (isEmpty()) last = first; else oldfirst.pre = first; N++; }          // insert the item at the front    public void addLast(Item item){ if(item == null) throw new java.lang.NullPointerException(); Node oldlast = last; last = new Node(); last.item = item; //last.next = null; last.pre=oldlast; if (isEmpty()) first = last; else oldlast.next = last; N++; }           // insert the item at the end    public Item removeFirst(){ if(isEmpty()) { throw new java.util.NoSuchElementException("Queue underflow"); } Item item=first.item; first=first.next; N--; if(isEmpty()) last=null; else first.pre=null; return item;  }                // delete and return the item at the front    public Item removeLast(){ if(isEmpty()) { throw new java.util.NoSuchElementException("Queue underflow"); } Item item=last.item; last=last.pre; N--; if(isEmpty()) first=null; else last.next=null; return item; }                 // delete and return the item at the end    public Iterator<Item> iterator(){   return new ListIterator();   }         // return an iterator over items in order from front to end    private class ListIterator implements Iterator<Item>{ private Node current = first; public boolean hasNext() { return current != null; } public void remove() { throw new java.lang.UnsupportedOperationException(); } public Item next() { if(!hasNext()) throw new java.util.NoSuchElementException(); Item item = current.item; current = current.next; return item; }  }   public static void main(String[] args){   Deque<String> deque = new Deque<String> ();   while(!StdIn.isEmpty()) {   String s = StdIn.readString();   if(!s.equals("-")) {   StdOut.println("1->deque.size()=" +deque.size());   deque.addFirst(s);   StdOut.println("2->deque.size()=" +deque.size());   }   else if(!deque.isEmpty()) {   StdOut.println(deque.removeFirst() + " ");   StdOut.println("3->deque.size()=" +deque.size());   }   }   StdOut.println("(" + deque.size() +" left on the deque)");   }   }   // unit testing

第二个类是随机队列

public class RandomizedQueue<Item> implements Iterable<Item> {   public RandomizedQueue()                 // construct an empty randomized queue   public boolean isEmpty()                 // is the queue empty?   public int size()                        // return the number of items on the queue   public void enqueue(Item item)           // add the item   public Item dequeue()                    // delete and return a random item   public Item sample()                     // return (but do not delete) a random item   public Iterator<Item> iterator()         // return an independent iterator over items in random order   public static void main(String[] args)   // unit testing}

这个类我是用数组实现的，因为这个类需要进行查找，用线性表显然浪费很多时间。
重点说一下
随机出队的思路：
就是随机抽取数组中的某一项，然后将队末元素赋值给这一项，同时将队末元素删除即可。
代码倒是不难不过我当时真的想了一段时间。

迭代器的代码还是要仔细学习一下，我个人认为以后用到的机会会很多。

源代码如下：

import java.util.Iterator;public class RandomizedQueue<Item> implements Iterable<Item> {    private Item[] s;private int N=0;public RandomizedQueue(){s = (Item[]) new Object[2];}                 // construct an empty randomized queue   public boolean isEmpty(){   return N==0;   }                 // is the queue empty?public int size(){   return N;   }                        // return the number of items on the queueprivate void resize(int capacity){Item[] temp = (Item[]) new Object[capacity];for (int i = 0; i < N; i++)temp[i] = s[i];s = temp;}public void enqueue(Item item){if(item == null) throw new java.lang.NullPointerException();if(N == s.length) resize(2*s.length);s[N++] = item;}           // add the item   public Item dequeue(){if(isEmpty()) throw new java.util.NoSuchElementException();int index = (int) (Math.random()*N);Item item = s[index];if(index != N-1) s[index] = s[N-1];s[N-1] = null;N--;if(N>0 && N==s.length/4) resize(s.length/2);return item;}                    // delete and return a random item   public Item sample(){if(isEmpty()) throw new java.util.NoSuchElementException();int index = (int) (Math.random()*N);Item item = s[index];return item;}                     // return (but do not delete) a random item   public Iterator<Item> iterator(){return new RQIterator();}         // return an independent iterator over items in random order   private class RQIterator implements Iterator<Item> {private int index = 0;private Item[] r;public RQIterator() {r = (Item[]) new Object[N];for(int i=0; i<N; i++)r[i] = s[i];StdRandom.shuffle(r);}public boolean hasNext() {return index < N;}public void remove() {throw new java.lang.UnsupportedOperationException();}public Item next() {if(!hasNext()) throw new java.util.NoSuchElementException();Item item = r[index++];return item;}}public static void main(String[] args){}   // unit testing}