Circular Queue | Set 2 (Circular Linked List Implementation)

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 rear=(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.



Below is the implementation of above approach:

C++
// 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;
}
Java
// Java program for insertion and
// deletion in Circular Queue
import java.util.*;

class Solution {

    // Structure of a Node
    static class Node {
        int data;
        Node link;
    }

    static class Queue {
        Node front, rear;
    }

    // Function to create Circular queue
    static void enQueue(Queue q, int value)
    {
        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
    static int deQueue(Queue q)
    {
        if (q.front == null) {
            System.out.printf("Queue is empty");
            return Integer.MIN_VALUE;
        }

        // If this is the last node to be deleted
        int value; // Value to be dequeued
        if (q.front == q.rear) {
            value = q.front.data;
            q.front = null;
            q.rear = null;
        }
        else // There are more than one nodes
        {
            Node temp = q.front;
            value = temp.data;
            q.front = q.front.link;
            q.rear.link = q.front;
        }

        return value;
    }

    // Function displaying the elements of Circular Queue
    static void displayQueue(Queue q)
    {
        Node temp = q.front;
        System.out.printf("\nElements in Circular Queue are: ");
        while (temp.link != q.front) {
            System.out.printf("%d ", temp.data);
            temp = temp.link;
        }
        System.out.printf("%d", temp.data);
    }

    /* Driver of the program */
    public static void main(String args[])
    {
        // 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
        System.out.printf("\nDeleted value = %d", deQueue(q));
        System.out.printf("\nDeleted value = %d", deQueue(q));

        // Remaining elements in Circular Queue
        displayQueue(q);

        enQueue(q, 9);
        enQueue(q, 20);
        displayQueue(q);
    }
}

// This code is contributed
// by Arnab Kundu
Python3
# Python3 program for insertion and
# deletion in Circular Queue

# Structure of a Node
class Node:
    def __init__(self):
        self.data = None
        self.link = None

class Queue:
    def __init__(self):
        front = None
        rear = None

# Function to create Circular queue
def enQueue(q, value):
    temp = Node()
    temp.data = value
    if (q.front == None):
        q.front = temp
    else:
        q.rear.link = temp

    q.rear = temp
    q.rear.link = q.front

# Function to delete element from
# Circular Queue
def deQueue(q):
    if (q.front == None):
        print("Queue is empty")
        return -999999999999

    # If this is the last node to be deleted
    value = None # Value to be dequeued
    if (q.front == q.rear):
        value = q.front.data
        q.front = None
        q.rear = None
    else: # There are more than one nodes
        temp = q.front
        value = temp.data
        q.front = q.front.link
        q.rear.link = q.front

    return value

# Function displaying the elements
# of Circular Queue
def displayQueue(q):
    temp = q.front
    print("Elements in Circular Queue are: ",
                                   end = " ")
    while (temp.link != q.front):
        print(temp.data, end = " ")
        temp = temp.link
    print(temp.data)

# Driver Code
if __name__ == '__main__':

    # Create a queue and initialize
    # front and rear
    q = Queue()
    q.front = q.rear = None

    # 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
    print("Deleted value = ", deQueue(q))
    print("Deleted value = ", deQueue(q))

    # Remaining elements in Circular Queue
    displayQueue(q)

    enQueue(q, 9)
    enQueue(q, 20)
    displayQueue(q)

# This code is contributed by PranchalK
C#
// C# program for insertion and
// deletion in Circular Queue
using System;
using System.Collections.Generic;

public class GFG {

    // Structure of a Node
    public class Node {
        public int data;
        public Node link;
    }

    public class LinkedList {
        public Node front, rear;
    }

    // Function to create Circular queue
    public static void enQueue(LinkedList q,
                               int value)
    {
        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
    public static int deQueue(LinkedList q)
    {
        if (q.front == null) {
            Console.Write("Queue is empty");
            return int.MinValue;
        }

        // If this is the last node to be deleted
        int value; // Value to be dequeued
        if (q.front == q.rear) {
            value = q.front.data;
            q.front = null;
            q.rear = null;
        }
        else // There are more than one nodes
        {
            Node temp = q.front;
            value = temp.data;
            q.front = q.front.link;
            q.rear.link = q.front;
        }

        return value;
    }

    // Function displaying the elements
    // of Circular Queue
    public static void displayQueue(LinkedList q)
    {
        Node temp = q.front;
        Console.Write("\nElements in Circular Queue are: ");
        while (temp.link != q.front) {
            Console.Write("{0:D} ", temp.data);
            temp = temp.link;
        }
        Console.Write("{0:D}", temp.data);
    }

    // Driver Code
    public static void Main(string[] args)
    {
        // Create a queue and initialize
        // front and rear
        LinkedList q = new LinkedList();
        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
        Console.Write("\nDeleted value = {0:D}",
                      deQueue(q));
        Console.Write("\nDeleted value = {0:D}",
                      deQueue(q));

        // Remaining elements in Circular Queue
        displayQueue(q);

        enQueue(q, 9);
        enQueue(q, 20);
        displayQueue(q);
    }
}

// This code is contributed by Shrikant13

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 the case of array implementation, we need a circular queue to save space.

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