Programming Assignment 2:Deques and Randomized Queues
Deque.java
// import java.util.Scanner; // Uncomment this line when using client testing
import java.util.Iterator;
public class Deque<Item> implements Iterable<Item> {
private Node first;
private Node last;
private int size;
private class Node {
Node next;
Node previous;
Item item;
}
public Deque() // construct an empty deque
{
first = null;
last = null;
size = 0;
}
public boolean isEmpty() // is the deque empty?
{ return size == 0; }
public int size() // return the number of items on the deque
{ return size; }
public void addFirst(Item item) // add the item to the front
{
if (item == null)
{ throw new java.lang.IllegalArgumentException(); }
Node oldFirst = first;
first = new Node();
first.item = item;
first.next = oldFirst;
first.previous = null;
if (isEmpty()) last = first;
else oldFirst.previous = first;
size++;
}
public void addLast(Item item) // add the item to the end
{
if (item == null)
{ throw new java.lang.IllegalArgumentException(); }
Node oldLast = last;
last = new Node();
last.item = item;
last.previous = oldLast;
last.next = null;
if (isEmpty()) first = last;
else oldLast.next = last;
size++;
}
public Item removeFirst() // remove and return the item from the front
{
if (isEmpty()) throw new java.util.NoSuchElementException();
Item item = first.item;
first = first.next;
if(first == null) last = null;
else first.previous = null;
size--;
return item;
}
public Item removeLast() // remove and return the item from the end
{
if (isEmpty()) throw new java.util.NoSuchElementException();
Item item = last.item;
last = last.previous;
if (last == null) first = null;
else last.next = null;
size--;
return item;
}
public Iterator<Item> iterator() // return an iterator over items in order from front to end
{ return new IteratorDeque(); }
private class IteratorDeque implements Iterator<Item> {
private Node current = first;
public boolean hasNext()
{ return current != null; }
public Item next()
{
if (!hasNext())
{ throw new java.util.NoSuchElementException(); }
Item item = current.item;
current = current.next;
return item;
}
public void remove()
{ throw new java.lang.UnsupportedOperationException(); }
}
public static void main(String[] args) // unit testing (optional)
{
System.out.println("client starts:");
// Deque<String> deque = new Deque<String>();
// Scanner scanner = new Scanner(System.in);
// String str;
// boolean ifFirst = false;
// Iterator<String> iterator = deque.iterator();
// while (true)
// {
// if (ifFirst) System.out.println("Operation on First:");
// else System.out.println("Operation on Last:");
// str = scanner.nextLine();
// if (str.compareTo("quit") == 0) break;
// if (str.compareTo("none") == 0)
// {
// ifFirst = !ifFirst;
// continue;
// }
// if (str.compareTo("-") == 0)
// {
// if (ifFirst) System.out.println(deque.removeFirst());
// else System.out.println(deque.removeLast());
// }
// else if (str.compareTo("count") == 0) System.out.println(deque.size());
// else if (str.compareTo("all") == 0)
// {
// System.out.println();
// for (String s : deque)
// { System.out.print(s+" "); }
// System.out.println();
// }
// else if (str.compareTo("null") == 0)
// {
// if (ifFirst) deque.addFirst(null);
// else deque.addLast(null);
// }
// else if (str.compareTo("iter") == 0)
// { iterator = deque.iterator(); }
// else if (str.compareTo("remove") == 0)
// { iterator.remove(); }
// else if (str.compareTo("next") == 0)
// { System.out.println(iterator.next()); }
// else
// {
// if (ifFirst) deque.addFirst(str);
// else deque.addLast(str);
// }
//
// }
// scanner.close();
}
}RandomizedQueue.java
思路：整体上类似栈的实现，取出前将栈顶元素与栈内部随机某个元素交换。
主类的dequeue()方法在实现这个交换时是直接对items[]数组交换的。将这种实现套用到iterator实现时，发现无法通过并行测试和嵌套测试。原因是每个iterator实例在调用next()方法的时候修改了它们共有的items[]数组，导致混乱，出现重复而无法遍历等问题。
因此改进为：每个iterator实例维护一个出栈顺序索引数组idx[]，初始化为0,1,2…N-1。每次出栈前对该数组进行前述交换，用idx[]上的交换替代以前那种对items[]具有破坏性的交换。这时每次调用next()方法，只需要返回索引值为idx[--current]的元素即可，即items[idx[--current]]。
进一步改进为：iterator实例初始化时就利用StdRandom.shuffle()把它打乱，免去了每次的交换。开销上应该是差不多的，但是代码更清晰。
// import java.util.Scanner; // Uncomment this line when using client testing
import java.util.Iterator;
import edu.princeton.cs.algs4.StdRandom;
public class RandomizedQueue<Item> implements Iterable<Item> {
private Item[] items;
private int N;
public RandomizedQueue() // construct an empty randomized queue
{
items = (Item[]) new Object[1];
N = 0;
}
public boolean isEmpty() // is the randomized queue empty?
{ return N == 0; }
public int size() // return the number of items on the randomized queue
{ return N; }
public void enqueue(Item item) // add the item
{
if (item == null) throw new java.lang.IllegalArgumentException();
if (items.length == N)
{ resize(2 * items.length); }
items[N++] = item;
}
private void resize(int newSize)
{
// System.out.println("Resizing from " + items.length + " to " + newSize); // Debugging info
Item copy[] = (Item[]) new Object[newSize];
for (int i = 0; i < N; i++)
{ copy[i] = items[i]; }
items = copy;
}
public Item dequeue() // remove and return a random item
{
if (isEmpty()) throw new java.util.NoSuchElementException();
swap(N-1, StdRandom.uniform(N));
Item item = items[--N];
items[N] = null;
if (N == items.length / 4 && N > 0) resize(items.length / 2);
return item;
}
private void swap(int idx1, int idx2)
{
Item temp = items[idx1];
items[idx1] = items[idx2];
items[idx2] = temp;
}
public Item sample() // return a random item (but do not remove it)
{
if (isEmpty()) throw new java.util.NoSuchElementException();
swap(N-1, StdRandom.uniform(N));
return items[N-1];
}
public Iterator<Item> iterator() // return an independent iterator over items in random order
{ return new QueueIterator(); }
private class QueueIterator implements Iterator<Item> {
private int current;
private int[] idx;
public QueueIterator()
{
current = N;
idx = new int[N];
for (int i = 0; i < N; i++)
{ idx[i] = i; }
StdRandom.shuffle(idx, 0, N);
}
// private void swapIndex(int i, int j)
// {
// int temp = idx[i];
// idx[i] = idx[j];
// idx[j] = temp;
// }
public boolean hasNext()
{ return current != 0; }
public Item next()
{
if (!hasNext()) throw new java.util.NoSuchElementException();
// swapIndex(current-1, StdRandom.uniform(current));
return items[idx[--current]];
}
public void remove()
{ throw new java.lang.UnsupportedOperationException(); }
}
public static void main(String[] args) // unit testing (optional)
{
System.out.println("client starts:");
// // Parallel test and nested test
// RandomizedQueue<String> queue1 = new RandomizedQueue<String>();
// for (int i = 1; i < 4; i++)
// { queue1.enqueue(i + ""); }
// RandomizedQueue<String> queue2 = new RandomizedQueue<String>();
// for (int i = 1; i < 4; i++)
// { queue2.enqueue(i + 3 + ""); }
// RandomizedQueue<String> queue3 = new RandomizedQueue<String>();
// for (int i = 1; i < 4; i++)
// { queue3.enqueue(i + 6 + ""); }
//
// RandomizedQueue<RandomizedQueue<String>> queueOfQueues = new RandomizedQueue<RandomizedQueue<String>>();
// queueOfQueues.enqueue(queue1);
// queueOfQueues.enqueue(queue2);
// queueOfQueues.enqueue(queue3);
//
// Iterator<String> iterator1 = queue1.iterator();
// Iterator<String> iterator2 = queue1.iterator();
//
// // Parallel test
// while (iterator1.hasNext())
// {
// System.out.print(iterator1.next() + " ");
// System.out.print(iterator2.next() + " \n");
// }
//
// // Nested test
// for (RandomizedQueue<String> queue : queueOfQueues)
// {
// System.out.println();
// for (String s : queue)
// { System.out.print(s + " "); }
// }
// // Interactive test
// Scanner scanner = new Scanner(System.in);
// RandomizedQueue<String> queue = new RandomizedQueue<String>();
// Iterator<String> iterator = queue.iterator();
// String str;
// while (true)
// {
// str = scanner.nextLine();
// if (str.compareTo("quit") == 0) break;
// if (str.compareTo("-") == 0)
// { System.out.println(queue.dequeue()); }
// else if (str.compareTo("sample") == 0)
// { System.out.println(queue.sample()); }
// else if (str.compareTo("all") == 0)
// {
// System.out.println();
// for (String s : queue) System.out.print(s + " ");
// System.out.println();
// }
// else if (str.compareTo("null") == 0)
// { queue.enqueue(null); }
// else if (str.compareTo("remove") == 0)
// {
// iterator = queue.iterator();
// iterator.remove();
// }
// else if (str.compareTo("iter") == 0)
// {
// iterator = queue.iterator();
// }
// else if (str.compareTo("next") == 0)
// {
// System.out.println(iterator.next());
// }
// else queue.enqueue(str);
// }
// scanner.close();
}
}Permutation.java
import edu.princeton.cs.algs4.StdIn;
import edu.princeton.cs.algs4.StdOut;
import java.util.Iterator;
public class Permutation {
public static void main(String[] args)
{
int k = Integer.parseInt(args[0]);
RandomizedQueue<String> queue = new RandomizedQueue<String>();
while (!StdIn.isEmpty())
{
queue.enqueue(StdIn.readString());
}
Iterator<String> iterator = queue.iterator();
for (int i = 0; i < k; i++)
{ StdOut.println(iterator.next()); }
}
}