Delete a Node from 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 without a null end.

intermediate

Why is deleting a node from a circular linked list different from a singly linked list?

Because the last node points back to the head, we must update the last node's next pointer if the head is deleted to maintain the circle.

beginner

In deleting a node from a circular linked list, what happens if the list has only one node?

Deleting the only node makes the list empty, so the head pointer becomes NULL.

intermediate

What is the general approach to delete a node with a given key in a circular linked list?

1. Find the node with the key.
2. If it's the head, update the last node's next pointer.
3. Remove the node by linking its previous node to its next node.
4. Free the deleted node.

beginner

What should you do if the node to delete is not found in the circular linked list?

No changes are made; the list remains the same because the key does not exist.

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

A The first node

B NULL

C Itself

D The previous node

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

When deleting the head node in a circular linked list, what must be updated?

A The tail pointer

B The last node's next pointer

C Nothing needs to be updated

D The head's previous pointer

The last node's next pointer must point to the new head to maintain the circular structure.

If a circular linked list has only one node and it is deleted, what is the new state of the list?

A List with two nodes

B List with one node pointing to itself

C Empty list (head is NULL)

D List remains unchanged

Deleting the only node empties the list, so the head becomes NULL.

What happens if the node to delete is not found in the circular linked list?

A The head node is deleted

B The last node is deleted

C The list becomes empty

D The list remains unchanged

If the key is not found, no deletion occurs and the list stays the same.

Which pointer is used to traverse a circular linked list during deletion?

A A temporary pointer starting at head

B A pointer starting at NULL

C A pointer starting at the last node

D No pointer is needed

A temporary pointer starting at head is used to find the node to delete.

Explain step-by-step how to delete a node with a given key from a circular linked list.

Think about how the circle is maintained after deletion.

Describe the special cases to consider when deleting nodes from a circular linked list.

Focus on how the list structure changes in these cases.

Practice

1. What happens when you delete the only node in a circular linked list?

easy

A. The list becomes empty (head = NULL)

B. The list still contains one node pointing to itself

C. The list becomes a linear linked list

D. The list throws an error

Solution

1. Step 1: Understand the structure of a single-node circular linked list

   It has one node whose next pointer points to itself.

2. Step 2: Delete the only node

   After deletion, no nodes remain, so the head pointer must be set to NULL to indicate an empty list.

3. Final Answer:

   The list becomes empty (head = NULL) -> Option A

4. Quick Check:

   Single node deleted means empty list [OK]

Hint: Deleting last node empties list, set head to NULL [OK]

Common Mistakes:

• Not setting head to NULL after deleting the only node
• Assuming list still has one node after deletion
• Confusing circular list with linear list after deletion

2. Which of the following is the correct syntax to delete the head node in a circular linked list in C?

easy

A. head = NULL; free(head);

B. head->next = NULL; free(head);

C. free(head); head = NULL;

D. Node* oldHead = head; head = head->next; free(oldHead);

Solution

1. Step 1: Understand deletion of head node in circular linked list

   We must move head pointer to next node and free the old head node.

2. Step 2: Check syntax correctness

   Node* oldHead = head; head = head->next; free(oldHead); correctly assigns head to head->next and frees the old head pointer stored separately.

3. Final Answer:

   Node* oldHead = head; head = head->next; free(oldHead); -> Option D

4. Quick Check:

   Move head, free old node [OK]

Hint: Move head to next, then free old head [OK]

Common Mistakes:

• Freeing head before moving pointer
• Setting next pointer to NULL breaks circular link
• Freeing NULL pointer

3. Given the circular linked list: 10 -> 20 -> 30 -> (back to 10), what is the list after deleting node with value 20?

medium

A. 10 -> 20 -> 30 -> (back to 10)

B. 20 -> 30 -> (back to 20)

C. 10 -> 30 -> (back to 10)

D. 30 -> 10 -> (back to 30)

Solution

1. Step 1: Identify node to delete and its neighbors

   Node with value 20 is between 10 and 30.

2. Step 2: Adjust pointers to remove node 20

   Set 10's next pointer to 30, skipping 20, maintaining circular link.

3. Final Answer:

   10 -> 30 -> (back to 10) -> Option C

4. Quick Check:

   Removed 20, linked 10 to 30 [OK]

Hint: Link previous node to next of deleted node [OK]

Common Mistakes:

• Not updating previous node's next pointer
• Breaking circular link by setting next to NULL
• Deleting wrong node

4. What is wrong with this code snippet for deleting a node with value 15 from a circular linked list?

void deleteNode(Node** head, int key) {
  Node* curr = *head;
  Node* prev = NULL;
  while (curr->data != key) {
    prev = curr;
    curr = curr->next;
  }
  prev->next = curr->next;
  free(curr);
}

medium

A. It does not handle deleting the head node correctly

B. It frees the wrong node

C. It causes infinite loop always

D. It sets head to NULL incorrectly

Solution

1. Step 1: Analyze deletion logic for head node

   If the node to delete is head, prev is NULL, so prev->next causes error.

2. Step 2: Identify missing head update

   Code does not update *head when deleting head node, causing incorrect list state.

3. Final Answer:

   It does not handle deleting the head node correctly -> Option A

4. Quick Check:

   Head deletion needs special handling [OK]

Hint: Check if node to delete is head before pointer update [OK]

Common Mistakes:

• Not updating head pointer when deleting head
• Dereferencing NULL prev pointer
• Not checking if node exists before deletion

5. You have a circular linked list with nodes: 5 -> 10 -> 15 -> 20 -> (back to 5). You want to delete the node with value 5 (head node). Which steps correctly delete the head and maintain the circular list?

hard

A. Set head to head->next, free old head, leave last node's next unchanged

B. Find last node, set its next to head->next, free head, update head to head->next

C. Free head, set head to NULL, list becomes empty

D. Set head->next to NULL, free head, update head to next node

Solution

1. Step 1: Locate last node in circular list

   Traverse nodes until node whose next points to head is found (node with value 20).

2. Step 2: Update last node's next pointer and head

   Set last node's next to head->next (node with value 10), free old head, and update head pointer to head->next.

3. Final Answer:

   Find last node, set its next to head->next, free head, update head to head->next -> Option B

4. Quick Check:

   Last node points to new head after deletion [OK]

Hint: Update last node's next before freeing head [OK]

Common Mistakes:

• Not updating last node's next pointer
• Freeing head before updating pointers
• Breaking circular link by setting next to NULL