Delete from Beginning of Doubly Linked List in DSA C# - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a doubly linked list?

A doubly linked list is a chain of nodes where each node has three parts: data, a pointer to the next node, and a pointer to the previous node. This allows moving forward and backward through the list.

beginner

What happens when you delete the first node from a doubly linked list?

The first node is removed, the head pointer moves to the second node, and the new head's previous pointer is set to null to disconnect from the old first node.

intermediate

Why do we set the new head's previous pointer to null after deletion?

Because the new head is now the first node, it should not point back to any node. Setting previous to null prevents incorrect backward links.

beginner

What should you check before deleting the first node in a doubly linked list?

You should check if the list is empty (head is null). If it is empty, there is nothing to delete.

intermediate

Show the C# code snippet to delete the first node from a doubly linked list.

if (head == null) return; // empty list Node temp = head; head = head.Next; if (head != null) head.Previous = null; temp = null; // optional for garbage collection

What pointer of the new head node must be updated after deleting the first node in a doubly linked list?

A Data pointer

B Previous pointer

C Next pointer

D Tail pointer

The previous pointer of the new head must be set to null to disconnect it from the old first node.

What should you do if the doubly linked list is empty and you try to delete the first node?

A Do nothing

B Throw an error

C Delete the tail node

D Create a new node

If the list is empty (head is null), there is no node to delete, so do nothing.

After deleting the first node, what becomes the new head of the doubly linked list?

A The second node

B The last node

C The deleted node

D Null

The second node becomes the new head after the first node is deleted.

Which of these is NOT a part of a doubly linked list node?

A Data

B Pointer to next node

C Pointer to previous node

D Pointer to random node

A standard doubly linked list node does not have a pointer to a random node.

In C#, what keyword can help remove references to a deleted node for garbage collection?

A delete

B free

C null

D remove

Setting the node reference to null helps the garbage collector know the object can be cleaned up.

Explain step-by-step how to delete the first node from a doubly linked list.

Think about what pointers need to change and what to do if the list is empty.

Write a simple C# method to delete the first node of a doubly linked list and explain each line.

Focus on pointer updates and safety checks.

Practice

1. What happens to the head pointer when you delete the first node from a doubly linked list?

easy

A. It becomes null immediately

B. It remains unchanged

C. It points to the last node

D. It moves to the second node in the list

Solution

1. Step 1: Understand the role of head pointer

   The head pointer always points to the first node in the doubly linked list.

2. Step 2: Effect of deleting the first node

   When the first node is deleted, the head pointer must move to the next node, which becomes the new first node.

3. Final Answer:

   It moves to the second node in the list -> Option D

4. Quick Check:

   Head points to new first node after deletion [OK]

Hint: Head always points to the first node after deletion [OK]

Common Mistakes:

• Assuming head becomes null even if list has more nodes
• Not updating head pointer after deletion
• Pointing head to last node mistakenly

2. Which of the following is the correct syntax to delete the first node in a doubly linked list in C#?

easy

A. head = head.next; head.prev = null;

B. head = head.prev; head.next = null;

C. head.next = null; head.prev = null;

D. head = null; head.next = null;

Solution

1. Step 1: Move head to next node

   To delete the first node, assign head to head.next to point to the second node.

2. Step 2: Remove backward link

   Set head.prev to null to disconnect the previous first node.

3. Final Answer:

   head = head.next; head.prev = null; -> Option A

4. Quick Check:

   Correct syntax updates head and prev pointer [OK]

Hint: Set head to next and prev of new head to null [OK]

Common Mistakes:

• Setting head.prev instead of head.next
• Not nullifying prev pointer of new head
• Setting head to null prematurely

3. Given the doubly linked list: 10 <-> 20 <-> 30 <-> null, what will be the list after deleting the first node?

medium

A. 10 <-> 30 <-> null

B. 20 <-> 30 <-> null

C. 30 <-> null

D. null

Solution

1. Step 1: Identify the first node

   The first node has value 10, which will be removed.

2. Step 2: Update head to second node

   Head moves to node with value 20, and its prev pointer is set to null.

3. Final Answer:

   20 <-> 30 <-> null -> Option B

4. Quick Check:

   First node removed, list starts at 20 [OK]

Hint: After deletion, head points to second node [OK]

Common Mistakes:

• Removing wrong node (like 20 or 30)
• Not updating prev pointer of new head
• Leaving old head in the list

4. Consider this code snippet to delete the first node from a doubly linked list:

if (head != null) {
    head = head.next;
    head.prev = null;
}

What error can occur here?

medium

A. Logical error: head should not be updated

B. Syntax error due to missing semicolon

C. NullReferenceException if list has only one node

D. No error, code is correct

Solution

1. Step 1: Analyze when list has one node

   If list has one node, head.next is null after assignment.

2. Step 2: Accessing head.prev causes error

   Setting head.prev = null when head is null causes NullReferenceException.

3. Final Answer:

   NullReferenceException if list has only one node -> Option C

4. Quick Check:

   Accessing property on null causes exception [OK]

Hint: Check if head is null before setting head.prev [OK]

Common Mistakes:

• Not checking if head is null after moving to next
• Assuming code always works for single-node list
• Ignoring runtime exceptions

5. You have a doubly linked list with nodes: 5 <-> 15 <-> 25 <-> 35 <-> null. After deleting the first node twice, what is the resulting list?

hard

A. 25 <-> 35 <-> null

B. 15 <-> 25 <-> 35 <-> null

C. 35 <-> null

D. null

Solution

1. Step 1: Delete first node once

   First deletion removes node with value 5; head moves to 15.

2. Step 2: Delete first node second time

   Second deletion removes node with value 15; head moves to 25.

3. Final Answer:

   25 <-> 35 <-> null -> Option A

4. Quick Check:

   Two deletions remove first two nodes [OK]

Hint: Each deletion moves head to next node [OK]

Common Mistakes:

• Deleting only once instead of twice
• Not updating head correctly after second deletion
• Confusing node values order