Circular Queue | Set 2 (Circular Linked List Implementation)

• Difficulty Level : Easy
• Last Updated : 30 Sep, 2020

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. Recommended: Please try your approach on {IDE} first, before moving on to the solution.

Below is the implementation of above approach:

C++

 // C++ program for insertion and// deletion in Circular Queue#include using namespace std;  // Structure of a Nodestruct Node {    int data;    struct Node* link;};  struct Queue {    struct Node *front, *rear;};  // Function to create Circular queuevoid 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 Queueint 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 Queuevoid 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 Queueimport 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 Codeif __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#

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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