Insert at End of Circular Linked List in DSA C - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a circular linked list?

A circular linked list is a linked list where the last node points back to the first node, forming a circle with no NULL end.

beginner

Why do we need a special method to insert at the end of a circular linked list?

Because the last node points to the first node, we must update the last node's next pointer to the new node and the new node's next pointer to the first node to keep the circle intact.

beginner

Inserting at the end of an empty circular linked list results in what?

The new node points to itself, making it both the first and last node in the list.

intermediate

What pointers are updated when inserting a new node at the end of a non-empty circular linked list?

The current last node's next pointer is updated to the new node, and the new node's next pointer is set to the head (first node).

intermediate

What is the time complexity of inserting at the end of a circular linked list if we do not maintain a tail pointer?

O(n), because we must traverse the list to find the last node before inserting.

What does the last node in a circular linked list point to?

A The previous node

B The first node

C Itself

D NULL

In a circular linked list, the last node points back to the first node to form a circle.

When inserting at the end of an empty circular linked list, what should the new node's next pointer be?

A Itself

B NULL

C The head node

D The previous node

The new node points to itself because it is the only node, forming a circle.

Which pointer must be updated to insert a new node at the end of a circular linked list?

A Both the last node's next pointer and the new node's next pointer

B The new node's next pointer only

C The last node's next pointer only

D The head pointer only

Both pointers must be updated to maintain the circular structure.

If you do not keep track of the last node, how do you find where to insert the new node at the end?

A Insert after the second node

B Insert at the head

C Traverse the list until the node whose next points to head

D Use a stack

You must traverse to find the last node whose next points to the head.

What is the time complexity of inserting at the end if a tail pointer is maintained?

A O(n^2)

B O(log n)

C O(n)

D O(1)

With a tail pointer, insertion at the end is done in constant time.

Explain step-by-step how to insert a new node at the end of a circular linked list.

Think about how the circle is maintained after insertion.

Describe the difference in insertion at the end of a circular linked list when a tail pointer is maintained versus when it is not.

Consider how knowing the last node helps.

Practice

1. What happens when you insert a new node at the end of a circular linked list that currently has 3 nodes?

easy

A. The new node becomes the head node and points to the second node.

B. The new node becomes the last node and points back to the head node.

C. The new node is inserted in the middle of the list.

D. The list breaks and becomes a linear linked list.

Solution

1. Step 1: Understand circular linked list structure

   In a circular linked list, the last node points back to the head node, forming a circle.

2. Step 2: Insert new node at end

   The new node is linked after the current last node, and its next pointer points to the head node, maintaining the circle.

3. Final Answer:

   The new node becomes the last node and points back to the head node. -> Option B

4. Quick Check:

   New node points to head, preserving circular link [OK]

Hint: New end node always points to head in circular list [OK]

Common Mistakes:

• Forgetting to link new node back to head
• Inserting node at wrong position
• Breaking the circular link

2. Which of the following is the correct syntax to create a new node in C for a circular linked list?

easy

A. struct Node* newNode = (struct Node*)malloc(sizeof(struct Node));

B. newNode = malloc(sizeof(Node));

C. struct Node newNode = malloc(sizeof(struct Node));

D. Node* newNode = new Node();

Solution

1. Step 1: Allocate memory for new node

   In C, use malloc with sizeof(struct Node) and cast to (struct Node*).

2. Step 2: Correct pointer declaration

   Declare newNode as a pointer to struct Node to hold the allocated memory address.

3. Final Answer:

   struct Node* newNode = (struct Node*)malloc(sizeof(struct Node)); -> Option A

4. Quick Check:

   Proper malloc and pointer cast in C [OK]

Hint: Use malloc with sizeof(struct Node) and cast to struct Node* [OK]

Common Mistakes:

• Missing (struct Node*) cast
• Declaring newNode as struct Node instead of pointer
• Using C++ style new operator in C

3. Given the following code snippet for inserting at the end of a circular linked list, what will be the printed list after insertion?

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

void printList(struct Node* head) {
  struct Node* temp = head;
  if (head != NULL) {
    do {
      printf("%d -> ", temp->data);
      temp = temp->next;
    } while (temp != head);
  }
  printf("NULL\n");
}

int main() {
  struct Node* head = (struct Node*)malloc(sizeof(struct Node));
  struct Node* second = (struct Node*)malloc(sizeof(struct Node));
  struct Node* third = (struct Node*)malloc(sizeof(struct Node));

  head->data = 1; head->next = second;
  second->data = 2; second->next = third;
  third->data = 3; third->next = head;

  // Insert new node with data 4 at end
  struct Node* newNode = (struct Node*)malloc(sizeof(struct Node));
  newNode->data = 4;
  newNode->next = head;
  third->next = newNode;

  printList(head);
  return 0;
}

medium

A. 1 -> 2 -> 3 -> 4 -> 1 -> ... (infinite loop)

B. 1 -> 2 -> 3 -> NULL

C. 3 -> 4 -> 1 -> 2 -> NULL

D. 1 -> 2 -> 3 -> 4 -> NULL

Solution

1. Step 1: Understand list before insertion

   Initial list is 1 -> 2 -> 3 -> back to 1 (circular).

2. Step 2: Insert new node at end

   New node with data 4 points to head; third node points to new node, extending list to 1 -> 2 -> 3 -> 4 -> back to 1.

3. Step 3: Print list

   printList prints nodes until it reaches head again, so output is 1 -> 2 -> 3 -> 4 -> NULL.

4. Final Answer:

   1 -> 2 -> 3 -> 4 -> NULL -> Option D

5. Quick Check:

   New node added at end, circular link maintained [OK]

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

Common Mistakes:

• Printing infinite loop by not stopping at head
• Forgetting to update last node's next pointer
• Incorrect print format

4. Identify the error in the following code snippet that attempts to insert a new node at the end of a circular linked list:

void insertEnd(struct Node** head, int data) {
  struct Node* newNode = (struct Node*)malloc(sizeof(struct Node));
  newNode->data = data;
  newNode->next = *head;

  struct Node* temp = *head;
  while (temp->next != NULL) {
    temp = temp->next;
  }
  temp->next = newNode;
}

medium

A. The function should not use double pointer for head.

B. newNode->next should be NULL instead of *head.

C. The while loop condition should check for temp->next != *head instead of NULL.

D. Memory allocation for newNode is missing.

Solution

1. Step 1: Analyze loop condition in circular list

   In a circular linked list, last node's next points to head, not NULL.

2. Step 2: Correct loop condition

   Loop should continue until temp->next equals head, not NULL, to find last node.

3. Final Answer:

   The while loop condition should check for temp->next != *head instead of NULL. -> Option C

4. Quick Check:

   Loop must detect circular end by comparing to head [OK]

Hint: In circular list, last node points to head, not NULL [OK]

Common Mistakes:

• Using NULL to detect end in circular list
• Setting newNode->next to NULL
• Not updating last node's next pointer

5. You have a circular linked list with only one node where head points to itself. You want to insert a new node at the end. Which sequence of steps correctly updates the list?

hard

A. Create new node, set newNode->next = head->next, set head->next = newNode.

B. Create new node, set newNode->next = head, set head->next = newNode, then update head to newNode.

C. Create new node, set newNode->next = NULL, set head->next = newNode.

D. Create new node, set newNode->next = newNode, set head = newNode.

Solution

1. Step 1: Understand single node circular list

   The single node has head->next pointing to itself (head).

2. Step 2: Insert at end

   Set newNode->next = head->next (which is head), then head->next = newNode. This makes head -> newNode -> head, maintaining circularity without changing head.

3. Final Answer:

   Create new node, set newNode->next = head->next, set head->next = newNode. -> Option A

4. Quick Check:

   head -> newNode -> head [OK]

Hint: For single node circular: newNode->next = head->next; head->next = newNode [OK]

Common Mistakes:

• Setting newNode->next to NULL breaking circularity
• Unnecessarily updating head to newNode
• Incorrectly linking new node to itself