Queues

What does the following method do: d.addFirst( ' a ' )

' a ' inserted at front

What does the following method do: d.offerFirst( ' b ' )

' b ' inserted at front

What does the following method do: d.offerLast( ' y ' )

' y ' inserted at rear

What does the following method do: d.addLast( ' z ' )

' z ' inserted at rear

Solve the following: 1 % 5 = 2 % 5 = 5 % 5 = 8 % 5 =

1 2 0 3

Fill in the blank: A circular array-based implementation of a queue can help achieve ______ time complexity for both insertion and removal operations.

A circular array-based implementation of a queue can help achieve constant (O(1)) time complexity for both insertion and removal operations. because it efficiently utilizes the array's space and avoids the need for shifting elements. In a circular array-based queue, two pointers (or indices) are maintained: one for the front of the queue and one for the rear. When an element is enqueued, it is placed at the rear index, and the rear index is updated to the next position. When an element is dequeued, it is removed from the front index, and the front index is updated to the next position. The circular aspect comes into play when the rear or front index reaches the end of the array. Instead of resizing or shifting elements, the indices wrap around to the beginning of the array, creating a circular effect. This approach ensures that both enqueue and dequeue operations take constant time, as the only operation required for each is updating the corresponding index. No resizing or shifting of elements is necessary, which would have incurred additional time complexity.

How do you implement the Queue Interface using a Circular array

A circular array-based implementation of a queue involves using an array and two pointers (front and rear). The elements are stored in a circular manner, so when the rear reaches the end of the array, it wraps around to the beginning.

What is a deque interface

A deque (double-ended queue) is a data structure that allows insertion and removal of elements from both ends. The Deque interface in Java provides methods to work with deques.

What is a Queue

A queue is a data structure that stores elements in a linear format, following a First-In-First-Out (FIFO) order. In Java, the Queue interface provides methods for insertion, removal, and accessing the front element of a queue.

How do you implement the Queue Interface using a SLL

A single-linked list-based implementation of a queue involves using a linked list, where the head of the list represents the front of the queue, and the tail represents the rear. To insert an element, you add it to the tail, and to remove an element, you remove the head.

What are the primary advantages and disadvantages of implementing a queue using a linked list versus a circular array?

Advantages and disadvantages of implementing a queue using a linked list vs. a circular array: Linked list implementation: Advantages:Dynamic size, no need to resize the underlying data structure Constant time complexity (O(1)) for both enqueue and dequeue operations Disadvantages:Increased space overhead due to references (pointers) stored in the nodes Circular array implementation: Advantages:More space-efficient compared to linked lists (no extra references needed) Constant time complexity (O(1)) for both enqueue and dequeue operations Disadvantages:Requires resizing the underlying array when it becomes full or empty Resizing can be an expensive operation (linear time complexity, O(n)) Understanding these trade-offs helps you choose the right data structure based on your specific use case and performance requirements.

What would be the front and rear indices of the circular array-based queue from the previous question after dequeuing an element?

After dequeuing an element, the front index would become 4, and the rear index would remain 7. This is because the front index is incremented by 1 and wrapped around using the modulo operator: front = (front + 1) % data.length => front = (3 + 1) % 10 => front = 4.

Consider the following circular array-based queue implementation: public class CircularQueue<E> { private int front, rear, size; private E[] data; public CircularQueue(int capacity) { front = rear = size = 0; data = (E[]) new Object[capacity]; } } If the current front index is 3, the current rear index is 7, the size of the queue is 5, and the array's length is 10, what would be the front and rear indices after enqueuing an element?

After enqueuing an element, the front index would remain 3, and the rear index would become 8. This is because the rear index is incremented by 1 and wrapped around using the modulo operator: rear = (rear + 1) % data.length => rear = (7 + 1) % 10 => rear = 8.

You are given a queue with the following elements: 5, 10, 15, 20, 25. Perform the following operations and provide the final state of the queue: a. Dequeue an element b. Enqueue the number 30 c. Dequeue two elements d. Enqueue the number 35 e. Peek the front element

After performing the given operations, the queue will look like this: [5, 10, 15, 20, 25]. Here's the step-by-step explanation: a. Dequeue an element: 5 is removed -> [10, 15, 20, 25] b. Enqueue the number 30: [10, 15, 20, 25, 30] c. Dequeue two elements: 10 and 15 are removed -> [20, 25, 30] d. Enqueue the number 35: [20, 25, 30, 35] e. Peek the front element: 20

T/F? A singly linked list allows traversal in both directions.

Answer: False Explanation: In a singly linked list, each node has only one pointer to the next node, which means you can only traverse in one direction. In a doubly linked list, each node has two pointers, one to the next node and another to the previous

T/F? Queues follow a First-In-First-Out (FIFO) order for data access.

Answer: True Explanation: Queues maintain elements in a linear order where the element that was added first will be the first to be removed. This principle is called First-In-First-Out (FIFO).

What Java Collections Framework provides two implementations of the Deque interface

ArrayDeque(uses a resizable circular array) & Linked List

True or False: In a Deque, Linked List supports indexed operations

False

True or False: The Queue does not implement the Iterable interface

False

True or False: When using a SLL to implement a Queue, Insertions are at the front of the queue and removals are the end

False Insertions are at the rear of a queue and removals are from the front

T/F? Stacks are generally more suitable than queues for managing print jobs in an operating system.

False. Queues are generally more suitable than stacks for managing print jobs in an operating system because they follow the FIFO order, ensuring that tasks are executed in the order they were generated.

If you need to implement a queue that handles tasks with different priorities, which data structure would be a better fit than a standard queue?

If you need to implement a queue that handles tasks with different priorities, a priority queue would be a better fit than a standard queue. A priority queue allows you to dequeue elements in order of priority rather than in the order they were enqueued.

Consider a queue implemented using a circular array. If the array is full and the front and rear indices are the same, how can you differentiate between an empty and a full queue?

In a queue implemented using a circular array, if the front and rear indices are the same and the array is full, you can differentiate between an empty and a full queue by maintaining a separate 'size' variable that keeps track of the number of elements in the queue. When the size is 0, the queue is empty; when the size is equal to the array length, the queue is full.

In a queue, the operation to add an element is called _____, and the operation to remove an element is called _____.

In a queue, the operation to add an element is called enqueue, and the operation to remove an element is called dequeue.

What is the primary difference between a stack and a queue in terms of element removal order?

In a stack, elements are removed in the Last-In, First-Out (LIFO) order, while in a queue, elements are removed in the First-In, First-Out (FIFO) order.

import java.util.Queue; import java.util.LinkedList; public class QueueExample { public static void main(String[] args) { Queue<String> queue = new LinkedList<>(); queue.add("A"); queue.add("B"); queue.add("C"); System.out.println(queue.poll()); System.out.println(queue.peek()); } }

Output: A B Explanation: The code creates a Queue and adds three elements: "A", "B", and "C". It then calls poll() to remove and print the first element ("A"), and peek() to print the next element without removing it ("B").

What does the following method do: d.pollFirst()

Removes front

What does the following method do: d.pollLast()

Removes rear

What does the following method do: d.peekFirst( )

Returns front

What does the following method do: d.peekLast( )

Returns rear

Explain what is happening // Java Code Queue q = newQueue(); q.enqueue(6); q.enqueue(4); q.enqueue(7); q.enqueue(3); q.enqueue(8); q.dequeue(); //front = 1 q.dequeue(); //front = 2 q.enqueue(9); Front = 0 count = 1 | | | | | | | 0 1 2 3 4 5 Front = 0 count = 1 | | | | | | | 0 1 2 3 4 5 Front = 2 count = 4 | 6 | 4 | 7 | 3 | 8 | 9 | 1 2 3 4 5 Front = 1 count = 5 | 6 | 4 | 7 | 3 | 8 | 9 | 1 2 3 4 0 Front = 2 count = 6 | 6 | 4 | 7 | 3 | 8 | 9 | 1 2 3 4 5

The code initializes a queue data structure and sets its front pointer to index 0 and its count to 1. The queue has a maximum capacity of 6 elements, and it is currently empty The Java code then enqueues the integers 6, 4, 7, 3, and 8 into the queue using the enqueue() method. The program then dequeues two elements from the front of the queue using the dequeue() method. This causes the front pointer to move forward in the queue by two positions. The front pointer is now pointing to the element with value 7. Next, the program enqueues the integer 9 into the queue using the enqueue() method. Since the queue is full, this causes the front pointer to wrap around to the beginning of the queue. The new front pointer is calculated as (0 + 1) % 6 = 1, which is the remainder when 1 is divided by 6. The front pointer is now pointing to the element with value 4. Finally, the program enqueues the integer 2 into the queue using the enqueue() method. The queue is full, so the front pointer wraps around to the beginning of the queue again. The new front pointer is calculated as (1 + 1) % 6 = 2, which is the remainder when 2 is divided by 6. The front pointer is now pointing to the element with value 7 again.

Explain what is happening // Java Code Queue q = new Queue(); q.enqueue(6); Front = 0 count = 1 | | | | | | | 0 1 2 3 4 5 Front = 0 count = 2 | 6 | | | | | | 0 1 2 3 4 5 insert item at (front + count) % items.length

The code initializes a queue data structure and sets its front pointer to index 0 and its count to 1. The queue has a maximum capacity of 6 elements, and it is currently empty. Then, the Java code then enqueues the integer value 6 into the queue using the enqueue() method. Since the queue is empty, the value 6 is added to the first (and only) element of the queue, at index 0. The front pointer remains at index 0, and the count is incremented to 2.

What are the disadvantages of using a single or double-linked list to implement a queue in terms of space efficiency?

The disadvantages of using a single or double-linked list to implement a queue in terms of space efficiency are the additional storage requirements for the references (pointers) stored in the nodes.

What is the purpose of using a circular array for implementing a queue, and how does it differ from a regular array implementation?

The purpose of using a circular array for implementing a queue is to achieve constant time complexity (O(1)) for both insertion and removal operations. In a regular array implementation, insertion at the rear and removal from the front can take linear time (O(n)) in the worst case, making it inefficient.

What is the time complexity of inserting an element at the front of a queue implemented using a single-linked list?

The time complexity of inserting an element at the front of a queue implemented using a single-linked list is O(1). Since we have a reference to the front node, we can quickly create a new node and update the front reference to point to the new node without traversing the list.

How do you properly implement a queue using a stack

To properly implement a queue using stacks, you will need to use two stacks. Here's how you can do it: 1. When enqueuing a data value, push it onto Stack 1 (the "input" stack). 2. When dequeuing a data value, you will first check if Stack 2 (the "output" stack) is empty or not. If Stack 2 is empty, pop all the elements from Stack 1 and push them onto Stack 2, reversing their order. Then, pop and return the top element from Stack 2. If Stack 2 is not empty, just pop and return the top element from Stack 2. By using two stacks in this manner, you can achieve the desired FIFO behavior of a queue. The enqueue operation will always add elements to the top of Stack 1, while the dequeue operation will remove elements from the top of Stack 2, maintaining the correct order.

Given a queue implemented with a circular array, provide a method for resizing the underlying array while preserving the order of the elements.

To resize a circular array-based queue, you can create a method like this: public void resize(int newCapacity) { E[] newData = (E[]) new Object[newCapacity]; for (int i = 0; i < size; i++) { newData[i] = data[(front + i) % data.length]; } data = newData; front = 0; rear = size; } This method creates a new array with the specified capacity, copies the elements from the original array to the new array in the correct order, updates the data reference to point to the new array, and sets the front and rear indices accordingly.

True or False: A deque can be used as a stack if elements are pushed and popped from the front of the deque

True

True or False: Storage space is increased when using a LL due to references stored in the nodes

True

True or False: The Queue interface implements the Collection interface

True

True or False: When using a SLL to implement a Queue, The number of elements in the queue is changed by methods insert and remove.

True

True or False: When using a SLL to implement a Queue, We need a reference to the last list node so that insertions can be performed at O(1)

True

True or False: a full implementation of Queue must implement all required methods of Collection (and the Iterable interface)

True

True or False: its possible to insert and remove items from a queue in constant time using a circular array

True

True or False: mod can be used to calculate the front and back positions in a circular array, therefore avoiding comparisons to the array size

True

T/F? In a queue, elements are removed in the same order they are inserted.

True. In a queue, elements are removed in the same order they are inserted (FIFO - First-In, First-Out).

T/F? When dequeueing an element from a queue implemented with a circular array, the front index is always incremented by 1.

True. When dequeueing an element from a queue implemented with a circular array, the front index is always incremented by 1, using the modulo operator to wrap around if necessary: front = (front + 1) % data.length. The reason for this is that the front index points to the first element in the queue, and when you dequeue an element, you remove the first element, so the front index needs to be updated to point to the new first element.

add(E e):

add(E e): Inserts the specified element into the queue, throwing an IllegalStateException if the queue is full.

What are the Deque Interface methods

addFirst(E e) and addLast(E e): Inserts the specified element at the front or rear of the deque, throwing an IllegalStateException if the deque is full. offerFirst(E e) and offerLast(E e): Inserts the specified element at the front or rear of the deque, returning true if successful, and false if the deque is full. removeFirst() and removeLast(): Retrieves and removes the first or last element of the deque, throwing a NoSuchElementException if the deque is empty. pollFirst() and pollLast(): Retrieves and removes the first or last element of the deque, returning null if the deque is empty. getFirst() and getLast(): Retrieves, but does not remove, the first or last element of the deque, throwing a NoSuchElementException if the deque is empty. peekFirst() and peekLast(): Retrieves, but does not remove, the first or last element of the deque, returning null if the deque is empty.

element()

element(): Retrieves, but does not remove, the head of the queue, throwing a NoSuchElementException if the queue is empty.

Implement the Queue Interface using a SLL

import java.util.LinkedList; import java.util.Queue; public class LinkedListQueue { public static void main( String[] args) { Queue<Integer> queue = new LinkedList<>(); // perform insertion, removal, ... } }

What is the purpose of the mod operator(%)

it is used to calculate remainders

Offer(E e)

offer(E e): Inserts the specified element into the queue if possible, returning true if successful, and false if the queue is full.

peek()

peek(): Retrieves, but does not remove, the head of the queue, returning null if the queue is empty.

poll()

poll(): Retrieves and removes the head of the queue, returning null if the queue is empty.

Implement the Queue Interface using a Circular array

public class CircularArrayQueue<E> { private int front, rear, size; private E[] data; public CircularArrayQueue(int capacity) { front = rear = size = 0; data = (E[]) new Object[capacity]; } public void enqueue(E e) { if (size == data.length) { throw new IllegalStateException("Queue is full"); } data[rear] = e; rear = (rear + 1) % data.length; size++; } public E dequeue() { if (isEmpty()) { throw new NoSuchElementException("Queue is empty"); } E e = data[front]; data[front] = null; front = (front + 1) % data.length; size--; return e; } public E peek() { if (isEmpty()) { throw new NoSuchElementException("Queue is empty"); } return data[front]; } public boolean isEmpty() { return size == 0; } }

Provide the missing code snippet to implement a circular array-based queue: public class CircularQueue<E> { private int front, rear, size; private E[] data; public CircularQueue(int capacity) { front = rear = size = 0; data = (E[]) new Object[capacity]; } public void enqueue(E e) { // Your code here } public E dequeue() { // Your code here } public boolean isEmpty() { // Your code here } public boolean isFull() { // Your code here } }

public void enqueue(E e) { if (isFull()) { throw new IllegalStateException("Queue is full"); } data[rear] = e; rear = (rear + 1) % data.length; size++; } public E dequeue() { if (isEmpty()) { throw new NoSuchElementException("Queue is empty"); } E e = data[front]; data[front] = null; front = (front + 1) % data.length; size--; return e; } public boolean isEmpty() { return size == 0; } public boolean isFull() { return size == data.length; }

remove()

remove(): Retrieves and removes the head of the queue, throwing a NoSuchElementException if the queue is empty.

what is the difference between Enqueue/Dequeue and add(), remove(), offer() and poll()

the main differences between these methods are in their behavior when handling edge cases, such as when the queue is full or empty. The offer() and poll() methods return a special value (either true/false or null) to indicate the outcome, while add() and remove() methods throw exceptions in such cases. in Java, the Queue interface uses the more general method names add and remove (as well as offer and poll) for enqueue and dequeue operations. These method names are shared with other collection interfaces in Java, such as List and Set, to provide a consistent API for interacting with various collection types.