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.- Create a new node dynamically and insert value into it.
- Check if front==NULL, if it is true then front = rear = (newly created node)
- If it is false then rare=(newly created node) and rear node always contains the address of the front node.

**Steps:****deQueue()**This function is used to delete an element from the circular queue. In a queue, the element is always deleted from front position.- Check whether queue is empty or not means front == NULL.
- If it is empty then display Queue is empty. If queue is not empty then step 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.

**Steps:**

// 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.

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