Delete from Beginning of Doubly Linked List in DSA Java - 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 two links: one to the next node and one 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 head pointer moves to the second node, and the new head's previous link is set to null. The original first node is removed from the list.

beginner

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

Because the head node has no node before it, its previous pointer must be null to mark the start of the list.

beginner

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

Check if the list is empty (head is null). If empty, no deletion is possible.

intermediate

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

if (head == null) return; // empty list head = head.next; if (head != null) head.prev = null;

What pointer do you update to remove the first node in a doubly linked list?

A Head pointer

B Tail pointer

C Next pointer of last node

D Previous pointer of last node

The head pointer must be updated to point to the second node to remove the first node.

After deleting the first node, what should the new head's previous pointer be set to?

A Null

B Head itself

C Next node

D Tail node

The new head has no previous node, so its previous pointer must be null.

What happens if you try to delete from an empty doubly linked list?

A New node is added

B List crashes

C Nothing happens

D Tail pointer changes

If the list is empty (head is null), deletion does nothing.

Which of these is NOT a step in deleting the first node of a doubly linked list?

A Set head to head.next

B Set tail to null

C Set new head.prev to null

D Check if list is empty

Setting tail to null is not required unless the list had only one node.

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

A Tail node

B Itself

C Next node

D Null

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

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

Think about what pointers need to change and what happens if the list is empty or has one node.

Write Java code to delete the first node of a doubly linked list and explain each line.

Focus on pointer updates and null checks.

Practice

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

easy

A. It moves to the second node in the list

B. It becomes null immediately

C. It points to the last node

D. It remains unchanged

Solution

1. Step 1: Understand the head pointer role

   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 A

4. Quick Check:

   Deleting first node updates head to next node [OK]

Hint: Head always points to first node; update it after deletion [OK]

Common Mistakes:

• Not updating head pointer after deletion
• Setting head to null even if list has more nodes
• Confusing head with tail pointer

2. Which of the following is the correct Java statement to update the prev pointer of the new head node after deleting the first node in a doubly linked list?

easy

A. head = null.prev;

B. head.next = null;

C. head.prev = null;

D. head.prev.next = null;

Solution

1. Step 1: Identify the new head node

   After deleting the first node, the new head is the second node in the list.

2. Step 2: Set the previous pointer of new head to null

   Since the new head is the first node now, its prev pointer must be null to indicate no node before it.

3. Final Answer:

   head.prev = null; -> Option C

4. Quick Check:

   New head prev pointer must be null [OK]

Hint: New head's prev pointer is always null [OK]

Common Mistakes:

• Setting head.next to null instead of head.prev
• Using invalid syntax like null.prev
• Trying to access prev.next which may cause error

3. Consider the following Java code snippet for deleting the first node from a doubly linked list:

if (head == null) return;
head = head.next;
if (head != null) head.prev = null;

What will be the state of the list after deleting the first node from the list 1 <-> 2 <-> 3?

medium

A. 1 <-> 3

B. Empty list

C. 3 only

D. 2 <-> 3

Solution

1. Step 1: Initial list and deletion

   The list is 1 <-> 2 <-> 3. Deleting the first node means removing node with value 1.

2. Step 2: Update head and prev pointer

   Head moves to node 2. Then head.prev is set to null, so node 2 has no previous node.

3. Final Answer:

   2 <-> 3 -> Option D

4. Quick Check:

   First node removed, head points to second node [OK]

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

Common Mistakes:

• Not setting head.prev to null causing broken links
• Assuming node 1 still exists
• Confusing next and prev pointers

4. The following code attempts to delete the first node from a doubly linked list but causes a runtime error:

if (head == null) return;
head = head.next;
head.prev = null;

What is the error and how to fix it?

medium

A. Syntax error due to missing semicolon; add semicolon after head.prev = null

B. NullPointerException when list has one node; fix by checking if head is null before setting head.prev

C. Logical error: head should not be updated; remove head = head.next

D. No error; code works fine

Solution

1. Step 1: Identify cause of NullPointerException

   If the list has only one node, after head = head.next, head becomes null. Then head.prev causes NullPointerException.

2. Step 2: Fix by checking head before accessing prev

   Add a condition to check if head is not null before setting head.prev = null.

3. Final Answer:

   NullPointerException when list has one node; fix by checking if head is null before setting head.prev -> Option B

4. Quick Check:

   Always check for null before accessing object members [OK]

Hint: Check head != null before head.prev = null to avoid errors [OK]

Common Mistakes:

• Not checking for null causing runtime errors
• Assuming list always has multiple nodes
• Ignoring exception details

5. You have a doubly linked list with nodes 10 <-> 20 <-> 30 <-> 40. After deleting the first node twice using the correct deletion method, what will be the printed list?

hard

A. 30 <-> 40

B. 20 <-> 30 <-> 40

C. 40 only

D. Empty list

Solution

1. Step 1: First deletion

   Deleting first node removes 10. Head moves to 20, head.prev set to null. List becomes 20 <-> 30 <-> 40.

2. Step 2: Second deletion

   Deleting first node again removes 20. Head moves to 30, head.prev set to null. List becomes 30 <-> 40.

3. Final Answer:

   30 <-> 40 -> Option A

4. Quick Check:

   Two deletions remove first two nodes [OK]

Hint: Each deletion moves head forward and updates prev to null [OK]

Common Mistakes:

• Deleting only once instead of twice
• Not updating head.prev after second deletion
• Printing old list without changes