Programming Assignment 2: Randomized Queues and Deques Solution

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Description

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 inserting 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

// is the deque

public boolean isEmpty()

empty?

// return the number

public int size()

of items on the deque

// insert the item

public void addFirst(Item item)

at the front

// insert the item

public void addLast(Item item)

at the end

// delete and return

public Item removeFirst()

the item at the front

// delete and return

public Item removeLast()

the item at the end

// return an

public Iterator<Item> iterator()

iterator over items in order from front to end

public static void main(String[] args)

// unit testing

}

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 proportional to 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?

// return the number

public int size()

of items on the queue

// add the item

public void enqueue(Item item)

public Item dequeue()

// delete and return

a random item

// return (but do

public Item sample()

not delete) a random item

// return an

public Iterator<Item> iterator()

independent iterator over items in random order

public static void main(String[] args)

// unit testing

}

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 use space proportional to 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 Subset 3

% echo AA BB BB

BB BB BB CC CC | java Subset 8

BB

C

G

AA

A

BB

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

CC

BB

E

BB

F

CC

G

BB

Your program must implement the following API:

public class Subset {

public static void main(String[] args)

}

You may assume that 0 ≤ kN, where N is the number of string on standard input. 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. You may not use any libraries other than those in java.lang, stdlib.jar, java.util.Iterator, and java.util.NoSuchElementException. Also submit the client program Subset.java. Finally, submit a readme.txt file and answer the questions.

This assignment was developed by Kevin Wayne.

Copyright © 2005.


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