Diving Deeper – Circular Queues and Deques
Now that we've got a solid understanding of Simple Queues, it's time to take the plunge and dive deeper into the world of Queues. In this module, we'll explore two other types of Queues – Circular Queues and Deques (Double Ended Queues).
Understanding Circular Queues
The Concept of a Circular Queue
In a Circular Queue, the last element points back to the first element making a circular link. We can visualize it as a circle where we remove elements from one end and add elements at the other end, and it goes on in a cycle.
Operations on a Circular Queue
We will use an array to implement circular queue.
The operations on a Circular Queue are similar to those of a Simple Queue – Enqueue, Dequeue, Peek, and IsEmpty. But there's a twist in the way we handle these operations due to the circular nature of the Queue.
When we Enqueue (add an element), we add it at the rear and increment the rear pointer. But if we reach the end of our array while enqueueing, instead of declaring an Overflow, we wrap around and continue from the front of the array, as long as there is space.
Similarly, when we Dequeue (remove an element), we remove it from the front and increment the front pointer. But if we reach the end of the array while dequeuing, we wrap around and continue from the start of the array.
class CircularQueue {
private int[] queue;
private int size;
private int front;
private int rear;
// Constructor to initialize the queue
public CircularQueue(int size) {
this.size = size;
queue = new int[this.size];
front = -1;
rear = -1;
}
// Function to insert an element in the queue
public void enqueue(int element) {
if (front == (rear + 1) % size) {
System.out.println("Queue is Full");
} else if (front == -1) {
// Insert First Element
front = 0;
rear = 0;
queue[rear] = element;
} else if (rear == size - 1 && front != 0) {
rear = 0;
queue[rear] = element;
} else {
rear = (rear + 1) % size;
queue[rear] = element;
}
}
// Function to delete an element from the queue
public int dequeue() {
if (front == -1) {
System.out.println("Queue is Empty");
return Integer.MIN_VALUE;
}
int data = queue[front];
queue[front] = -1;
if (front == rear) {
front = -1;
rear = -1;
} else if (front == size - 1) {
front = 0;
} else {
front++;
}
return data;
}
// Function to display the elements of the queue
public void displayQueue() {
if (front == -1) {
System.out.println("Queue is Empty");
return;
}
System.out.println("Elements in the Circular Queue are: ");
if (rear >= front) {
for (int i = front; i <= rear; i++) {
System.out.print(queue[i] + " ");
}
} else {
for (int i = front; i < size; i++) {
System.out.print(queue[i] + " ");
}
for (int i = 0; i <= rear; i++) {
System.out.print(queue[i] + " ");
}
}
System.out.println();
}
}
public class Solution {
// Main method to test the CircularQueue class
public static void main(String[] args) {
CircularQueue q = new CircularQueue(5);
// Inserting elements in the queue
q.enqueue(14);
q.enqueue(22);
q.enqueue(13);
q.enqueue(-6);
// Display elements present in the queue
q.displayQueue();
// Deleting elements from the queue
System.out.println("Deleted value = " + q.dequeue());
System.out.println("Deleted value = " + q.dequeue());
q.displayQueue();
q.enqueue(9);
q.enqueue(20);
q.enqueue(5);
q.displayQueue();
q.enqueue(20);
}
}
Common Mistakes and How to Avoid Them
While working with Queues, it's easy to make a few common mistakes. The first mistake is forgetting to check for Queue Overflow and Underflow conditions. Always remember to check if the Queue is full before enqueueing and if it's empty before dequeuing. This will save you from unexpected errors.
Another common mistake is mixing up the front and rear pointers. Keep in mind that we always add items to the rear and remove them from the front.
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