Create a Circular Singly Linked List in DSA C

Mental Model

A circular singly linked list is like a chain where the last link connects back to the first, making a loop.

Analogy: Imagine a round table where each person holds hands with the next, and the last person holds hands with the first, so everyone forms a circle.

head -> [1] -> [2] -> [3] ->
          ↑               ↓
          ← ← ← ← ← ← ← ← ←

Dry Run Walkthrough

Input: Create a circular singly linked list with nodes 1, 2, 3 in order

Goal: Build a linked list where the last node points back to the first node, forming a circle

Step 1: Create node with value 1 and set head to it

head -> [1] -> null

Why: Start the list with the first node

Step 2: Create node with value 2 and link it after node 1

head -> [1] -> [2] -> null

Why: Add second node to the list

Step 3: Create node with value 3 and link it after node 2

head -> [1] -> [2] -> [3] -> null

Why: Add third node to the list

Step 4: Make node 3 point back to head node 1

head -> [1] -> [2] -> [3] -> head (1)

Why: Close the loop to form a circular list

Result:

head -> [1] -> [2] -> [3] -> head (1)

Annotated Code

DSA C

#include <stdio.h>
#include <stdlib.h>

// Node structure for circular singly linked list
typedef struct Node {
    int data;
    struct Node* next;
} Node;

// Function to create a circular singly linked list with given array of values
Node* createCircularList(int arr[], int n) {
    if (n == 0) return NULL; // empty list

    Node* head = (Node*)malloc(sizeof(Node));
    head->data = arr[0];
    Node* curr = head;

    // Create and link nodes for remaining elements
    for (int i = 1; i < n; i++) {
        Node* newNode = (Node*)malloc(sizeof(Node));
        newNode->data = arr[i];
        curr->next = newNode; // link current node to new node
        curr = newNode;       // move current pointer forward
    }

    curr->next = head; // last node points back to head to form circle
    return head;
}

// Function to print circular singly linked list
void printCircularList(Node* head) {
    if (head == NULL) {
        printf("List is empty\n");
        return;
    }

    Node* curr = head;
    do {
        printf("%d -> ", curr->data);
        curr = curr->next;
    } while (curr != head);
    printf("(back to head)\n");
}

int main() {
    int values[] = {1, 2, 3};
    int n = sizeof(values) / sizeof(values[0]);

    Node* circularList = createCircularList(values, n);
    printCircularList(circularList);

    return 0;
}

if (n == 0) return NULL; // empty list

handle empty input array by returning NULL

head->data = arr[0];

initialize head node with first element

curr->next = newNode; // link current node to new node

link previous node to new node to extend list

curr = newNode; // move current pointer forward

advance current pointer to newly added node

curr->next = head; // last node points back to head to form circle

close the list by linking last node back to head

do { ... } while (curr != head);

traverse and print nodes until we loop back to head

Output

1 -> 2 -> 3 -> (back to head)

Complexity Analysis

Time: O(n) because we create and link each of the n nodes once

Space: O(n) because we allocate memory for n nodes

vs Alternative: Compared to a linear singly linked list, circular list requires one extra pointer assignment to close the loop, but traversal can be infinite if not careful

Edge Cases

empty input array

function returns NULL and prints 'List is empty'

DSA C

if (n == 0) return NULL; // empty list

single element array

creates one node that points to itself forming a single-node circle

DSA C

curr->next = head; // last node points back to head to form circle

When to Use This Pattern

When you need a list where you can cycle through elements repeatedly without stopping, use a circular singly linked list to avoid null ends.

Common Mistakes

Mistake: Forgetting to link the last node back to the head, leaving the list linear
Fix: Add the line 'curr->next = head;' after creating all nodes to close the circle

Mistake: Using a while loop that checks for NULL to print nodes, causing infinite loop in circular list
Fix: Use a do-while loop that stops when current node returns to head

Summary

Creates a linked list where the last node points back to the first, forming a circle.

Use when you want to cycle through elements endlessly without a null end.

The key is linking the last node back to the head to close the loop.

Practice

1. What is a key characteristic of a circular singly linked list compared to a regular singly linked list?

easy

A. The last node points back to the first node, forming a circle.

B. Each node has two pointers, one to the next and one to the previous node.

C. It uses an array internally to store nodes.

D. The list ends with a null pointer indicating the end.

Solution

1. Step 1: Understand the pointer structure in circular singly linked list

   In a circular singly linked list, the last node's next pointer points back to the first node, creating a loop.

2. Step 2: Compare with regular singly linked list

   Regular singly linked lists end with a null pointer, while circular ones loop back to the start.

3. Final Answer:

   The last node points back to the first node, forming a circle. -> Option A

4. Quick Check:

   Circular list last node points to head = C [OK]

Hint: Remember: circular means last node links back to first [OK]

Common Mistakes:

• Thinking circular list ends with null pointer
• Confusing circular singly linked list with doubly linked list
• Assuming nodes have two pointers in singly linked list

2. Which of the following is the correct way to initialize a circular singly linked list with one node in C?

easy

A. node->next = head->next;

B. node->next = NULL;

C. node->next = node;

D. node->next = 0;

Solution

1. Step 1: Understand initialization of single node circular list

   For a circular singly linked list with one node, the node's next pointer must point to itself to form a circle.

2. Step 2: Check each option

   node->next = node; sets node->next = node, correctly creating a self-loop. Others either set null or incorrect pointers.

3. Final Answer:

   node->next = node; -> Option C

4. Quick Check:

   Single node points to itself = A [OK]

Hint: Single node circular list points to itself [OK]

Common Mistakes:

• Setting next pointer to NULL instead of self
• Using uninitialized pointers
• Confusing NULL with 0 in pointer context

3. Consider the following C code snippet that creates a circular singly linked list with three nodes. What will be the output of the print loop?

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

int main() {
  Node n1 = {1, NULL};
  Node n2 = {2, NULL};
  Node n3 = {3, NULL};

  n1.next = &n2;
  n2.next = &n3;
  n3.next = &n1;  // circular link

  Node* temp = &n1;
  int count = 0;
  while (count < 5) {
    printf("%d ", temp->data);
    temp = temp->next;
    count++;
  }
  return 0;
}

medium

A. 1 2 3 3 3

B. 1 2 3 2 3

C. 1 2 3 1 1

D. 1 2 3 1 2

Solution

1. Step 1: Trace the linked list nodes and their next pointers

   n1 points to n2, n2 points to n3, n3 points back to n1, forming a circle: 1 -> 2 -> 3 -> 1 ...

2. Step 2: Follow the print loop for 5 iterations

   Print order: 1 (n1), 2 (n2), 3 (n3), 1 (n1 again), 2 (n2 again).

3. Final Answer:

   1 2 3 1 2 -> Option D

4. Quick Check:

   Loop prints nodes in circle: 1 2 3 1 2 [OK]

Hint: Follow next pointers in circle for given count [OK]

Common Mistakes:

• Stopping after 3 nodes instead of 5
• Not following circular link back to first node
• Confusing node data values

4. The following code attempts to create a circular singly linked list with two nodes. What is the error?

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

int main() {
  Node* n1 = malloc(sizeof(Node));
  Node* n2 = malloc(sizeof(Node));
  n1->data = 10;
  n2->data = 20;
  n1->next = n2;
  n2->next = NULL;  // mistake here
  return 0;
}

medium

A. Memory allocation is missing for nodes.

B. n2->next should point to n1 to make the list circular.

C. Data values are not initialized correctly.

D. n1->next should be NULL instead of n2.

Solution

1. Step 1: Identify the circular link requirement

   In a circular singly linked list, the last node's next pointer must point back to the first node.

2. Step 2: Check the code's next pointers

   n1->next points to n2, but n2->next is NULL, breaking the circle. It should point to n1.

3. Final Answer:

   n2->next should point to n1 to make the list circular. -> Option B

4. Quick Check:

   Last node next points to head = D [OK]

Hint: Last node's next must link to first node [OK]

Common Mistakes:

• Setting last node next to NULL instead of head
• Forgetting to allocate memory for nodes
• Mixing up which node points where

5. You want to insert a new node with data 5 at the end of an existing circular singly linked list. The list currently has nodes 1 -> 2 -> 3 -> (back to 1). Which steps correctly insert the new node to maintain the circular structure?

hard

A. Create new node; set new_node->next = head; find last node; set last_node->next = new_node;

B. Create new node; set new_node->next = NULL; find last node; set last_node->next = new_node;

C. Create new node; set new_node->next = last_node; set head = new_node;

D. Create new node; set new_node->next = head->next; set head->next = new_node;

Solution

1. Step 1: Create new node and link it to head

   The new node's next pointer must point to the head to keep the circle intact.

2. Step 2: Find the last node and update its next pointer

   Traverse the list to find the last node (which points to head), then set last_node->next to new_node.

3. Final Answer:

   Create new node; set new_node->next = head; find last node; set last_node->next = new_node; -> Option A

4. Quick Check:

   New node next = head, last node next = new node = A [OK]

Hint: New node next points to head; last node next points to new node [OK]

Common Mistakes:

• Setting new node next to NULL breaks circularity
• Not updating last node's next pointer
• Confusing head and last node pointers