COS 226 Programming Assignment

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 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
   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
}

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.

Your deque implementation should support each deque operation in constant worst-case time and use space proportional to the number of items currently in the deque. Additionally, your iterator implementation should support each iteration operation (including construction) in constant worst-case time, and use a constant amount of extra space per iterator.

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()              // 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
}

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.

Your randomized queue implementation should 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) should take at most cM steps in the worst case, for some constant c. Additionally, your iterator implementation should support each iteration operation (excluding construction) in constant worst-case time, and use a linear amount of extra space per iterator. The order of two iterators to the same randomized queue should be independent; each iterator must maintain its own random order.

Clients. Write a client program to solve each of the following problems. Use one variable of either type Deque or RandomizedQueue in each client. Use generics to avoid casting in your clients.

• Write a 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. You may assume that k ≥ 0 and no greater than the number of string on standard input.

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

• Write a program Palindrome.java that reads in a sequence of characters from standard input, using StdIn.readChar(), and determines whether that sequence represents a Watson-Crick complemented palindrome: the {A, C, T, G}-sequence equals its reverse when you replace each base with its complement: A-T, C-G). Watson-Crick palindromes have many important biological roles. For example, tumor cells frequently amplify their genes by forming Watson-Crick palindromes.

  % echo AAAACGTTTT | java Palindrome            % echo AAAACTTTT | java Palindrome
  true                                           false
  
  % echo AGCTAGCT | java Palindrome              % echo agctagct | java Palindrome
  true                                           false
  
  % echo GC | java Palindrome                    % echo AGCTTCGA | java Palindrome
  true                                           false
  
  % echo | java Palindrome                       % echo A | java Palindrome
  true                                           false
  

Deliverables. Submit the data types RandomizedQueue.java and Deque.java. Each data type should include its own main() that thoroughly tests the associated operations. You may not call any library functions other than those in stdlib.jar and Integer.parseInt(). Also submit the client programs Subset.java and Palindrome.java. Finally, submit a readme.txt file and answer the questions.