Circular & Circular Doubly Linked Lists in C++: Concepts & Code

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Introduction

A circular linked list is a variation of the linked list in C++ where the last node points back to the first node, forming a continuous loop. This structure allows traversal of the list without reaching a NULL pointer. A doubly circular linked list extends this concept further by allowing movement in both forward and backward directions, with the last node linked to the first and the first linked to the last. These data structures are especially useful in applications such as task scheduling, playlists, and round-robin algorithms. In this blog post, we’ll explore how circular and doubly circular linked lists work in C++ and why they are important in data structure design.

Let’s Explore Circular and Circular Doubly Linked Lists in C++

Today, we are going to use Circular Linked List and Doubly Circular Linked List in C++.

Circular Linked List

A circular linked list is a variation of a singly linked list in which the last node does not point to NULL. Instead, it points back to the first node, forming a circle. Because of this structure, traversal can start from any node and continue indefinitely until the starting node is reached again. Circular linked lists are useful in applications that require repeated looping, such as round-robin CPU scheduling or multiplayer turn-based games. They use less memory than doubly linked lists because each node contains only one pointer. Ultimately, the code will look like this:

#include <iostream>
using namespace std;

struct Node {
    int data;
    Node* next;
};

int main() {
    Node *head = NULL, *temp, *newNode;

    // create 3 nodes
    for (int i = 1; i <= 3; i++) {
        newNode = new Node();
        newNode->data = i * 10;

        if (head == NULL) {
            head = newNode;
            newNode->next = head;
            temp = head;
        } else {
            temp->next = newNode;
            newNode->next = head;
            temp = newNode;
        }
    }

    // display
    temp = head;
    do {
        cout << temp->data << " ";
        temp = temp->next;
    } while (temp != head);

    return 0;
}

Now, here's the overall code to understand it better:

As you can see in the above images, the code creates a circular linked list where the last node points to the first node. The insert() function adds nodes at the end, and the display() function prints all elements using a do-while loop until it reaches the head again.

Doubly Circular Linked List

A doubly circular linked list is an extension of a doubly linked list where both ends are connected circularly. In this structure, the next pointer of the last node points to the first node, and the prev pointer of the first node points to the last node. This allows traversal in both forward and backward directions without encountering a NULL pointer. Although this structure requires extra memory due to the additional pointer, it offers greater flexibility and efficiency for operations like insertion, deletion, and reverse traversal. Ultimately, the code will look like this:

#include <iostream>
using namespace std;

struct Node {
    int data;
    Node* next;
    Node* prev;
};

int main() {
    Node *head = NULL, *temp, *newNode;

    // create 3 nodes
    for (int i = 1; i <= 3; i++) {
        newNode = new Node();
        newNode->data = i * 10;

        if (head == NULL) {
            head = newNode;
            head->next = head;
            head->prev = head;
            temp = head;
        } else {
            newNode->next = head;
            newNode->prev = temp;
            temp->next = newNode;
            head->prev = newNode;
            temp = newNode;
        }
    }

    // display
    temp = head;
    do {
        cout << temp->data << " ";
        temp = temp->next;
    } while (temp != head);

    return 0;
}

Now, here's the overall code to understand it better:

As you can see in the above images, the code creates a doubly circular linked list where each node has next and prev pointers. The insert() function links the new node between the last node and the head, and the display() function prints the list by traversing once around the circle.

Difference Between Circular and Doubly Circular Linked List

The main difference between a circular linked list and a doubly circular linked list lies in the number of pointers per node and traversal capability. A circular linked list uses a single pointer and supports traversal in only one direction, making it simpler and more memory-efficient. In contrast, a doubly circular linked list uses two pointers per node, enabling bidirectional traversal but requiring additional memory. Both structures eliminate NULL pointers and are designed for applications that need continuous or cyclic data access.

Important Notes (Things Beginners Often Miss with Circular & Doubly Circular Linked Lists in C++)

One of the most common mistakes beginners make with circular linked lists is forgetting the stopping condition during traversal. Since these lists do not end with NULL, using a normal while (temp != NULL) loop will cause an infinite loop. Always use a do-while loop or explicitly stop when the pointer reaches the head again.

Another frequent issue is incorrect pointer linking during insertion. In a circular linked list, the last node must always point back to the head. If this link is missed even once, the circular structure breaks. In a doubly circular linked list, both next and prev pointers must be updated correctly for the head and the last node, otherwise backward traversal will fail.

Edge cases are especially important here. Beginners often forget to handle situations like inserting the first node, deleting the only node, or updating the head node. When the list has just one node, that node’s next and prev should point to itself. Missing this detail can cause runtime errors.

Memory management is another overlooked area. Since nodes are created dynamically using new, they must be properly deleted when removed from the list. Failing to do so leads to memory leaks, which become harder to debug in circular structures due to their looping nature.

Finally, beginners sometimes assume doubly circular linked lists are always better. While they offer two-way traversal and flexibility, they also consume more memory and require more careful pointer handling. Choosing between circular and doubly circular lists should depend on the problem requirements. Practicing pointer diagrams and tracing each link step by step will help you master these structures with confidence.

Conclusion

Circular and doubly circular linked lists provide efficient ways to manage data that needs continuous traversal. By eliminating the NULL end condition, they make looping operations smoother and more flexible. Doubly circular linked lists add even more power by enabling two-way navigation. Understanding these linked list variations helps strengthen your knowledge of pointers and dynamic memory structures, preparing you for more complex algorithms and real-world programming challenges in C++. With practice, these data structures become valuable tools in efficient program design.

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