Implement queue using linked list

In this problem, we need to implement a queue data structure using a linked list. A queue is a linear data structure that follows the First-In-First-Out (FIFO) principle, meaning that the element added first will be removed first. In a queue, elements are added to the back (enqueue operation) and removed from the front (dequeue operation).

Problem Statement

The problem is to implement a queue data structure using a linked list and provide basic operations like enqueue, dequeue, peek (to view the front element), size (to get the number of elements in the queue), and isEmpty (to check if the queue is empty).

Idea to Solve the Problem

To implement the queue using a linked list, we can use a class QNode to represent each node in the linked list. The QNode contains an integer data value and a reference to the next node in the linked list. We also need a class MyQueue to represent the queue, which contains a reference to the head and tail of the linked list, as well as a count to keep track of the number of elements in the queue.

Pseudocode

Class QNode
    Integer data
    QNode next
    QNode(value)
        data = value
        next = null

Class MyQueue
    QNode head
    QNode tail
    Integer count
    MyQueue()
        head = null
        tail = null
        count = 0

    Integer size()
        Return count

    Boolean isEmpty()
        Return count == 0

    Void enqueue(Integer value)
        Create a new QNode with value as data
        If head is null
            Set head to the new node
        Else
            Set next of tail to the new node
        Set tail to the new node
        Increment count

    Integer dequeue()
        If head is null
            Print "Empty Queue"
            Return -1
        Set temp to head
        Set head to next of head
        Decrement count
        If head is null
            Set tail to null
        Return data of temp

    Integer peek()
        If head is null
            Print "Empty Queue"
            Return -1
        Return data of head

Main function
    Create an instance of MyQueue called task
    Print "isEmpty: " + task.isEmpty()
    Enqueue elements 10, 20, 30, 40, 50 into the queue
    Print "size: " + task.size()
    Print "peek: " + task.peek()
    Print "dequeue: " + task.dequeue()
    Print "size: " + task.size()
    Print "peek: " + task.peek()
    Print "isEmpty: " + task.isEmpty()

Algorithm

1. Create a class QNode to represent each node in the linked list. The class contains an integer data value and a reference to the next node in the linked list.
2. Create a class MyQueue to represent the queue. The class contains references to the head and tail of the linked list, as well as a count to keep track of the number of elements in the queue.
3. Define a constructor for the MyQueue class to initialize the head and tail as null and the count as 0.
4. Implement the size() method to return the count of elements in the queue.
5. Implement the isEmpty() method to check if the queue is empty by checking if the count is 0.
6. Implement the enqueue(Integer value) method to add a new element to the queue. Create a new QNode with the given value, and if the head is null, set it as the head. Otherwise, set the next of the tail to the new node. Set the tail to the new node and increment the count.
7. Implement the dequeue() method to remove and return the front element of the queue. If the head is null, print "Empty Queue" and return -1. Otherwise, set a temporary variable temp to the head, set the head to the next of the head, decrement the count, and if the head is null, set the tail to null. Return the data of the temp node.
8. Implement the peek() method to return the front element of the queue without removing it. If the head is null, print "Empty Queue" and return -1. Otherwise, return the data of the head node.
9. In the main function, create an instance of MyQueue called task.
10. Print "isEmpty: " + task.isEmpty() to check if the queue is initially empty.
11. Enqueue elements 10, 20, 30, 40, and 50 into the queue using the enqueue method.
12. Print "size: " + task.size() to get the number of elements in the queue.
13. Print "peek: " + task.peek() to view the front element of the queue.
14. Print "dequeue: " + task.dequeue() to remove and return the front element of the queue.
15. Print "size: " + task.size() to get the updated number of elements in the queue.
16. Print "peek: " + task.peek() to view the front element of the queue after dequeueing.
17. Print "isEmpty: " + task.isEmpty() to check if the queue is empty after dequeueing.

Code Solution

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Time Complexity

The time complexity of enqueue and dequeue operations in a queue implemented using a linked list is O(1) since adding or removing elements from the back or front of the linked list takes constant time. The time complexity of other operations like peek, size, and isEmpty is also O(1) as they involve simple operations on variables.