Circular Linked List

Definition

A circular linked list is a linked list in which the last node’s link field points to the first node, creating a circular chain of nodes instead of ending with a null reference.

In a circular linked list:

Every node contains data and a pointer/link to the next node.
The last node links back to the head node.
Traversal from any node can continue around the circle until the starting node is encountered again.

If head is the first node and tail is the last node, then:

tail->next = head

This is the defining property of a circular linked list.

Main Content

1. First Concept: Structure of a Circular Linked List

A circular linked list is built from nodes, and each node contains:
Data part: stores the actual value
Link part: stores the address of the next node
Unlike a simple linear linked list, the last node points back to the first node, creating a closed chain.
Because of this circular connection, there is no NULL at the end of the list.
A circular linked list may be:
Singly circular linked list: each node points to the next node, and the last node points to the first node
Doubly circular linked list: each node has both next and prev pointers, and the first and last nodes are connected in both directions

Example of a singly circular linked list:

[5] -> [12] -> [18] -> [25]
 ^                       |
 |_______________________|

This means:

5 is the first node
25 is the last node
25 points back to 5

2. Second Concept: Traversal in a Circular Linked List

Traversal means visiting each node exactly once in a proper order.
In a circular linked list, traversal cannot rely on NULL as the stopping condition because the list does not end with NULL.
Instead, traversal usually starts at the head node and continues until the traversal reaches the head again.
A common method is:
Print/process the first node
Move to the next node
Keep moving until the current node becomes equal to the starting node again
Care must be taken to avoid an infinite loop

Example traversal flow:

Start at head
   ↓
Node 1 → Node 2 → Node 3 → Node 4
   ↑                         ↓
   └─────────────────────────┘

If the traversal starts at Node 1, it should stop when Node 1 is encountered again after Node 4.

Important observation:

Circular traversal is useful when repeated cycling through the same nodes is needed.
It is commonly used in scheduling tasks where each process gets a turn one after another.

3. Third Concept: Insertion and Deletion Operations

Insertion

means adding a new node to the circular linked list.

Deletion

means removing an existing node from the list.
These operations require updating pointers carefully so the circular structure remains valid.

Insertion cases:

Insert at beginning

New node points to the current head
Last node points to the new node
Head is updated to the new node

Insert at end

New node points to the head
Old last node points to the new node

Insert in middle

Adjust links of neighboring nodes so the new node fits between them

Deletion cases:

Delete from beginning

Head moves to the next node
Last node’s link is updated to the new head

Delete from end

Find the second-last node
Make it point to the head

Delete a specific node

Search for the node
Bypass it by changing the previous node’s link

Example of inserting 15 between 10 and 20:

Before:
[10] -> [20] -> [30]
 ^                 |
 |_________________|

After:
[10] -> [15] -> [20] -> [30]
 ^                           |
 |___________________________|

Correct pointer handling is essential; otherwise, the circular chain may break or form a wrong loop.

Working / Process

1. Create the first node

Allocate memory for the first node.
Store data in it.
Make its next pointer refer to itself if it is the only node in the list.
This self-reference is a valid circular form for a single-node circular linked list.

2. Add more nodes

For each new node, allocate memory and store the data.
Link the new node to the first node or next appropriate node.
Update the last node so that its pointer always refers back to the head node.

3. Traverse or modify the list

Start from the head node.
Visit each node in order.
Stop when the starting node is reached again.
For insertion or deletion, carefully change the links so the circle remains complete.

Example of a single-node circular list:

[40]
  |
  └── points to itself

Example of a three-node circular list:

[10] -> [20] -> [30]
 ^                 |
 |_________________|

This process ensures the structure remains continuous and usable for repeated cycles.

Advantages / Applications

Efficient cyclic traversal

Ideal when the data must be visited repeatedly in a loop.
Useful in applications like process scheduling, where each task gets repeated turns.

No need to restart from the beginning

Since the last node connects to the first node, traversal can continue naturally without special reset logic.
This makes implementation convenient in circular queues and round-robin systems.

Useful in real-world systems

Commonly used in:
- Round-robin CPU scheduling
- Circular buffers
- Multiplayer games and turn-based applications
- Repeating playlists
- Network token passing
Also helpful when data needs to be maintained in a continuous cycle.

Summary

Circular linked list is a linked list in which the last node points back to the first node.
It supports continuous traversal without a null ending.
It is especially useful for repeated or cyclic processing tasks.
Important terms to remember
Node
Head
Tail
Link pointer
Traversal
Insertion
Deletion
Singly circular linked list
Doubly circular linked list