Circular Queue | Set 2 (Circular Linked List Implementation)

1

Prerequisite – Circular Singly Linked List

We have discussed basics and how to implement circular queue using array in set 1.
Circular Queue | Set 1 (Introduction and Array Implementation)

In this post another method of circular queue implementation is discussed, using Circular Singly Linked List.

Operations on Circular Queue:

  • Front:Get the front item from queue.
  • Rear: Get the last item from queue.
  • enQueue(value) This function is used to insert an element into the circular queue. In a circular queue, the new element is always inserted at Rear position.
      Steps:

    1. Create a new node dynamically and insert value into it.
    2. Check if front==NULL, if it is true then front = rear = (newly created node)
    3. If it is false then rare=(newly created node) and rear node always contains the address of the front node.
  • deQueue() This function is used to delete an element from the circular queue. In a queue, the element is always deleted from front position.
      Steps:

    1. Check whether queue is empty or not means front == NULL.
    2. If it is empty then display Queue is empty. If queue is not empty then step 3
    3. Check if (front==rear) if it is true then set front = rear = NULL else move the front forward in queue, update address of front in rear node and return the element.
  • Operations-on-Circular -Queue

    // C or C++ program for insertion and
    // deletion in Circular Queue
    #include <bits/stdc++.h>
    using namespace std;
    
    // Structure of a Node
    struct Node
    {
        int data;
        struct Node* link;
    };
    
    struct Queue
    {
        struct Node *front, *rear;
    };
    
    // Function to create Circular queue
    void enQueue(Queue *q, int value)
    {
        struct Node *temp = new Node;
        temp->data = value;
        if (q->front == NULL)
            q->front = temp;
        else
            q->rear->link = temp;
    
        q->rear = temp;
        q->rear->link = q->front;
    }
    
    // Function to delete element from Circular Queue
    int deQueue(Queue *q)
    {
        if (q->front == NULL)
        {
            printf ("Queue is empty");
            return INT_MIN;
        }
    
        // If this is the last node to be deleted
        int value; // Value to be dequeued
        if (q->front == q->rear)
        {
            value = q->front->data;
            free(q->front);
            q->front = NULL;
            q->rear = NULL;
        }
        else  // There are more than one nodes
        {
            struct Node *temp = q->front;
            value = temp->data;
            q->front = q->front->link;
            q->rear->link= q->front;
            free(temp);
        }
    
        return value ;
    }
    
    // Function displaying the elements of Circular Queue
    void displayQueue(struct Queue *q)
    {
        struct Node *temp = q->front;
        printf("\nElements in Circular Queue are: ");
        while (temp->link != q->front)
        {
            printf("%d ", temp->data);
            temp = temp->link;
        }
        printf("%d", temp->data);
    }
    
    /* Driver of the program */
    int main()
    {
        // Create a queue and initialize front and rear
        Queue *q = new Queue;
        q->front = q->rear = NULL;
    
        // Inserting elements in Circular Queue
        enQueue(q, 14);
        enQueue(q, 22);
        enQueue(q, 6);
    
        // Display elements present in Circular Queue
        displayQueue(q);
    
        // Deleting elements from Circular Queue
        printf("\nDeleted value = %d", deQueue(q));
        printf("\nDeleted value = %d", deQueue(q));
    
        // Remaining elements in Circular Queue
        displayQueue(q);
    
        enQueue(q, 9);
        enQueue(q, 20);
        displayQueue(q);
    
        return 0;
    }
    

    Output:

    Elements in Circular Queue are: 14 22 6
    Deleted value = 14
    Deleted value = 22
    Elements in Circular Queue are: 6
    Elements in Circular Queue are: 6 9 20
    

    Time Complexity: Time complexity of enQueue(), deQueue() operation is O(1) as there is no loop in any of the operation.

    Note : In case of linked list implementation, a queue can be easily implemented without being circular. However in case of array implementation, we need a circular queue to save space.

    This article is contributed by Akash Gupta. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

    Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

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