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. There is no null at the 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 first node, we must update the links carefully to keep the circle intact after deletion.

beginner

In a circular linked list, what happens if you delete the only node present?

The list becomes empty, so the head pointer should be set to null to indicate no nodes remain.

beginner

What is the first step when deleting a node with a given value in a circular linked list?

Find the node to delete by traversing the list, keeping track of the previous node to update links.

intermediate

How do you update the links when deleting the head node in a circular linked list?

Find the last node, set its next pointer to the new head (head's next), then update the head pointer to head's next.

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 a node from a circular linked list, what must you update?

A The previous node's next pointer

B The node's data

C The head pointer only

D Nothing

You must update the previous node's next pointer to skip the deleted node and maintain the circle.

If the circular linked list has only one node and you delete it, what should the head be set to?

A Itself

B The deleted node

C Null

D The next node

Deleting the only node means the list is empty, so head should be null.

Which node's pointer must be updated when deleting the head node in a circular linked list?

A The second node's next pointer

B The last node's next pointer

C The head node's next pointer

D No pointers need updating

The last node's next pointer must point to the new head after deletion.

What is the time complexity to delete a node in a circular linked list if you have only the head pointer?

A O(1)

B O(n^2)

C O(log n)

D O(n)

You may need to traverse the list to find the node and its previous node, which takes O(n) time.

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

Think about how the circle is maintained after removal.

Describe how the circular linked list structure changes when the head node is deleted.

Focus on the pointers that keep the circle intact.

Practice

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

easy

A. The list becomes a linear linked list.

B. The list still contains one node pointing to itself.

C. The list becomes empty (null).

D. The list throws an error.

Solution

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

   In a circular linked list with one node, the node points to itself.

2. Step 2: Delete the only node

   Removing this node leaves no nodes, so the list becomes empty (null).

3. Final Answer:

   The list becomes empty (null). -> Option C

4. Quick Check:

   Deleting the only node empties the list [OK]

Hint: Deleting last node empties the list, no nodes remain [OK]

Common Mistakes:

• Assuming list still has one node after deletion
• Thinking list becomes linear instead of empty
• Expecting an error on deletion

2. Which of the following is the correct way to update the head pointer when deleting the head node in a circular linked list?

easy

A. head = head.next;

B. head = null;

C. head = head.prev;

D. head = head.next.next;

Solution

1. Step 1: Identify the new head after deleting the current head

   When deleting the head node, the new head should be the next node in the list.

2. Step 2: Update the head pointer

   Assign head to head.next to point to the next node.

3. Final Answer:

   head = head.next; -> Option A

4. Quick Check:

   New head is next node after deletion [OK]

Hint: After deleting head, move head pointer to next node [OK]

Common Mistakes:

• Setting head to null incorrectly
• Using head.prev which is invalid in singly circular list
• Skipping updating head pointer

3. What will be the output after deleting node with value 3 from the circular linked list: 1 -> 2 -> 3 -> 4 -> (back to 1)?

Node head = new Node(1);
head.next = new Node(2);
head.next.next = new Node(3);
head.next.next.next = new Node(4);
head.next.next.next.next = head;
// Delete node with value 3
// After deletion, print list starting from head

medium

A. 2 -> 3 -> 4 -> (back to 2)

B. 1 -> 2 -> 3 -> 4 -> (back to 1)

C. 1 -> 2 -> (back to 1)

D. 1 -> 2 -> 4 -> (back to 1)

Solution

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

   Node with value 3 is after node with value 2.

2. Step 2: Update previous node's next pointer

   Set node 2's next to node 4, skipping node 3.

3. Final Answer:

   1 -> 2 -> 4 -> (back to 1) -> Option D

4. Quick Check:

   Node 3 removed, list links 2 to 4 [OK]

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

Common Mistakes:

• Not updating previous node's next pointer
• Deleting wrong node
• Breaking circular link

4. Given this code snippet to delete a node from a circular linked list, what is the bug?

Node current = head;
while (current.next != head && current.next.data != key) {
    current = current.next;
}
if (current.next.data == key) {
    current.next = current.next.next;
}

medium

A. Does not handle deleting the head node correctly.

B. Does not update head pointer after deletion.

C. Deletes multiple nodes instead of one.

D. Infinite loop if key not found.

Solution

1. Step 1: Analyze deletion logic

   The code finds the node before the target and skips the target node.

2. Step 2: Check head pointer update

   If the deleted node is head, head pointer is not updated, causing inconsistency.

3. Final Answer:

   Does not update head pointer after deletion. -> Option B

4. Quick Check:

   Head pointer must update if head node deleted [OK]

Hint: Always update head if deleted node is head [OK]

Common Mistakes:

• Ignoring head pointer update
• Assuming deletion always non-head
• Not checking if key exists

5. You have a circular linked list with nodes: 10 -> 20 -> 30 -> 40 -> (back to 10). You want to delete the node with value 10 (head node). Which steps correctly delete the head and maintain the circular structure?

hard

A. Find last node, set its next to head.next, update head to head.next.

B. Set head to null directly.

C. Set head to head.next without changing last node's next.

D. Remove head and leave last node pointing to null.

Solution

1. Step 1: Find the last node in the circular list

   Traverse nodes until node.next == head to find last node (node with value 40).

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

   Set last node's next to head.next (node 20), then update head to head.next.

3. Final Answer:

   Find last node, set its next to head.next, update head to head.next. -> Option A

4. Quick Check:

   Last node points to new head, list remains circular [OK]

Hint: Update last node's next and move head to next node [OK]

Common Mistakes:

• Not updating last node's next pointer
• Setting head to null incorrectly
• Breaking circular link by pointing to null