Programming Assignment 2:  Randomized Queues and Deques

Write a generic data type for a deque and a randomized queue. The goal of this assignment is to implement elementary data structures using arrays and linked lists, and to introduce you to generics and iterators.

Dequeue. A double-ended queue or deque (pronounced "deck") is a generalization of a stack and a queue that supports adding and removing items from either the front or the back of the data structure. Create a generic data type Deque that implements the following API:

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)          // add the item to the front
   public void addLast(Item item)           // add the item to the end
   public Item removeFirst()                // remove and return the item from the front
   public Item removeLast()                 // remove and return the item from 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 (required)
}

Corner cases.  Throw a java.lang.NullPointerException if the client attempts to add a null item; throw a java.util.NoSuchElementException if the client attempts to remove an item from an empty deque; throw a java.lang.UnsupportedOperationException if the client calls the remove() method in the iterator; throw a java.util.NoSuchElementException if the client calls the next() method in the iterator and there are no more items to return.

Performance requirements.  Your deque implementation must support each deque operation (including construction) in constant worst-case time and use space linear in the number of items currently in the deque. Additionally, your iterator implementation must support each operation (including construction) in constant worst-case time.

Randomized queue. A randomized queue is similar to a stack or queue, except that the item removed is chosen uniformly at random from items in the data structure. Create a generic data type RandomizedQueue that implements the following API:

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()                    // remove and return a random item
   public Item sample()                     // return a random item (but do not remove it)
   public Iterator<Item> iterator()         // return an independent iterator over items in random order
   public static void main(String[] args)   // unit testing (required)
}

Corner cases.  The order of two or more iterators to the same randomized queue must be mutually independent; each iterator must maintain its own random order. Throw a java.lang.NullPointerException if the client attempts to add a null item; throw a java.util.NoSuchElementException if the client attempts to sample or dequeue an item from an empty randomized queue; throw a java.lang.UnsupportedOperationException if the client calls the remove() method in the iterator; throw a java.util.NoSuchElementException if the client calls the next() method in the iterator and there are no more items to return.

Performance requirements.  Your randomized queue implementation must support each randomized queue operation (besides creating an iterator) in constant amortized time and and use space linear in the number of items currently in the queue. That is, any sequence of M randomized queue operations (starting from an empty queue) must take at most cM steps in the worst case, for some constant c. Additionally, your iterator implementation must support next() and hasNext() in constant worst-case time and construction in linear time; you may use a linear amount of extra memory per iterator.

Client. Write a client program Subset.java that takes a command-line integer k; reads in a sequence of strings from standard input using StdIn.readString(); and prints out exactly k of them, uniformly at random. Each item from the sequence can be printed out at most once.

% echo A B C D E F G H I | java-algs4 Subset 3
C
G
A

% echo A B C D E F G H I | java-algs4 Subset 3
E
D
G

% echo AA BB BB BB BB BB CC CC | java-algs4 Subset 8
BB
AA
BB
CC
BB
BB
CC
BB

public class Subset {
   public static void main(String[] args)
}

Performance requirements:  The running time of Subset must be linear in the size of the input. You may use only a constant amount of memory plus either one Deque or RandomizedQueue object of maximum size at most N. (For an extra challenge, limit the maximum size to k.)

Deliverables. Submit the data types RandomizedQueue.java and Deque.java. Each of these two data type must include its own main() that thoroughly tests the associated operations. Also submit the client program Subset.java. Your submission may not call library functions except those in StdIn, StdOut, StdRandom, java.lang, java.util.Iterator, and java.util.NoSuchElementException. In particular, you may not use either java.util.LinkedList or java.util.ArrayList. Finally, submit a readme.txt file and answer the questions.