Delete a Node from Circular Linked List in DSA C# - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check if the list is empty before deletion.

DSA C#

if (head == [1]) {
    return null;
}

Drag options to blanks, or click blank then click option'

Aempty

Bhead

Cthis

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Checking if head equals head (always true).

Using 'empty' which is not a keyword.

We check if head is null to know if the list is empty before deleting a node.

2fill in blank

medium

Complete the code to find the node before the head in the circular linked list.

DSA C#

Node current = head;
while (current.next != [1]) {
    current = current.next;
}

Drag options to blanks, or click blank then click option'

Ahead

Bnull

Ccurrent

Dtail

Attempts:

3 left

💡 Hint

Common Mistakes

Checking current.next != null which never happens in circular list.

Using an undefined variable like tail.

In a circular linked list, the last node points back to head, so we check current.next != head.

3fill in blank

hard

Fix the error in the code to delete the head node correctly.

DSA C#

if (head.data == key) {
    if (head.next == head) {
        head = [1];
    } else {
        Node last = head;
        while (last.next != head) {
            last = last.next;
        }
        last.next = head.next;
        head = head.next;
    }
    return head;
}

Drag options to blanks, or click blank then click option'

Ahead

Bnull

Cthis

Dhead.next

Attempts:

3 left

💡 Hint

Common Mistakes

Setting head to head.next which causes a loop.

Using 'this' which is invalid here.

When the list has only one node and it matches the key, we set head to null to delete it.

4fill in blank

hard

Fill both blanks to delete a node with given key that is not the head.

DSA C#

Node current = head;
while (current.next != head && current.next.data != [1]) {
    current = current.next;
}
if (current.next.data == [2]) {
    current.next = current.next.next;
}

Drag options to blanks, or click blank then click option'

Akey

Bhead

Ccurrent

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Using head or current instead of key for data comparison.

Not checking if node with key exists before deletion.

We look for the node whose next node has the key, then skip it by changing next pointer.

5fill in blank

hard

Fill all three blanks to complete the delete function for circular linked list.

DSA C#

public Node DeleteNode(Node head, int key) {
    if (head == [1]) return null;
    Node current = head;
    if (head.data == key) {
        if (head.next == head) {
            head = [2];
        } else {
            Node last = head;
            while (last.next != head) {
                last = last.next;
            }
            last.next = head.next;
            head = [3].next;
        }
        return head;
    }
    while (current.next != head && current.next.data != key) {
        current = current.next;
    }
    if (current.next.data == key) {
        current.next = current.next.next;
    }
    return head;
}

Drag options to blanks, or click blank then click option'

Anull

Chead

Dthis

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'this' instead of null.

Not updating head after deleting the first node.

We check if head is null, set head to null if only one node is deleted, and update head to next node after deletion.

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