ReverseFirstKElementsOfQueue.java

/*
 * Author   : Hasnain Memon
 * Date     : 22/11/2024
 */
class Queue {

    private Node front;
    private Node rear;
    private int size;

    public Queue() {
        front = rear = null;
        size = 0;
    }

    static class Node {

        int data;
        Node next;

        public Node(int data) {
            this.data = data;
            next = null;
        }
    }

    public boolean isEmpty() {

        return (front == null);
    }

    public void enqueue(int data) {

        Node newNode = new Node(data);
        if (isEmpty()) {
            front = rear = newNode;
            size++;
            return;
        }
        rear.next = newNode;
        rear = newNode;
        size++;
    }

    public int dequeue() {

        int temp = front.data;
        if (isEmpty()) {
            System.out.println("Queue is empty!");
            return -1;
        }
        // only one element
        if (front.next == null) {
            rear = null;
        }
        // more than one element
        front = front.next;
        size--;
        return temp;
    }

    public void displayQueue() {

        if (isEmpty()) {
            System.out.println("Queue is empty!");
            return;
        }
        System.out.print("Queue: ");
        Node current = front;
        while (current != null) {
            System.out.print(current.data + " ");
            current = current.next;
        }
        System.out.println();
    }

    public int getSize() {

        return size;
    }

    // Task 1: Reverse the first K elements of a Queue.
    public void reverseFirstElements(int k) {

        if (isEmpty()) {
            System.out.println("Queue is empty!");
            return;
        }
        if (k < 0 || k > size) {
            System.out.println("Invalid value of k!");
            return;
        }

        Node current = front, next = null, previous = null;
        for (int i = 0; i < k && current != null; i++) {
            next = current.next;
            current.next = previous;
            previous = current;
            current = next;
        }
        if (front != null) {
            front.next = current;
        }
        front = previous;
    }
}

public class ReverseFirstKElementsOfQueue {

    public static void main(String[] args) {

        Queue queue = new Queue();

        queue.enqueue(10);
        queue.enqueue(5);
        queue.enqueue(30);
        queue.enqueue(45);
        queue.enqueue(60);
        queue.enqueue(2);

        queue.displayQueue();

        System.out.println("Size = " + queue.getSize());

        // Task 1
        queue.reverseFirstElements(4);
//        queue.reverseFirstElements(3);
//        queue.reverseFirstElements(queue.getSize());

        queue.displayQueue();
    }
}