Create a Circular Singly Linked List in DSA C - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a Circular Singly Linked List?

A Circular Singly Linked List is a linked list where the last node points back to the first node, forming a circle. There is no NULL at the end.

beginner

How do you identify the last node in a Circular Singly Linked List?

The last node is the one whose next pointer points back to the head (first node) instead of NULL.

intermediate

Why use a Circular Singly Linked List instead of a regular singly linked list?

It allows continuous traversal from any node without stopping, useful for applications like round-robin scheduling or buffering.

beginner

What is the key step when creating the first node in a Circular Singly Linked List?

The first node's next pointer must point to itself to form the circle when it is the only node.

beginner

In C, what structure is typically used to represent a node in a Circular Singly Linked List?

A struct with at least two fields: one for data (e.g., int) and one pointer to the next node.

In a Circular Singly Linked List, what does the last node's next pointer point to?

A Itself

B NULL

C The first node

D Random memory

The last node points back to the first node to form a circle.

When creating the first node in a Circular Singly Linked List, what should its next pointer be set to?

A Itself

B Random address

C The second node

D NULL

The first node points to itself to maintain the circular structure when it is alone.

Which of these is NOT a benefit of a Circular Singly Linked List?

A Useful for round-robin scheduling

B No need to check for NULL to end traversal

C Easy continuous traversal

D Faster random access to elements

Circular lists do not provide faster random access; they still require traversal.

What is the main difference between a singly linked list and a circular singly linked list?

A Circular lists have two pointers per node

B Circular lists have last node pointing to head

C Singly linked lists have no data

D Circular lists use arrays internally

The last node in a circular singly linked list points back to the head node.

In C, which keyword is used to define the node structure for a Circular Singly Linked List?

A struct

B class

C union

D typedef

The 'struct' keyword defines a node with data and a pointer to the next node.

Explain how to create the first node of a Circular Singly Linked List in C.

Think about how to make the list circular with only one node.

Describe the difference between a regular singly linked list and a circular singly linked list.

Focus on the pointer of the last node.

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