Delete from Beginning of Doubly Linked List in DSA Python - 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 traversal in both directions.

beginner

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

The head pointer moves to the second node, and the new head's previous pointer is set to None. The old first node is removed from the list.

intermediate

Why do we need to update the previous pointer of the new head to null after deletion?

Because the new head is now the first node, it should not point back to any node. Setting its previous pointer to None ensures the list starts correctly.

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 None). If it is empty, there is nothing to delete.

intermediate

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

def delete_from_beginning(head): if head is None: return None new_head = head.next if new_head is not None: new_head.prev = None return new_head

What pointer must be updated when deleting the first node in a doubly linked list?

A The next pointer of the new head

B The previous pointer of the new head

C The data pointer of the first node

D The next pointer of the last node

After deletion, the new head's previous pointer must be set to None to mark it as the first node.

What should happen if the doubly linked list is empty and you try to delete from the beginning?

A Delete the last node

B Throw an error

C Return the list as is (None head)

D Create a new node

If the list is empty (head is None), deletion does nothing and returns None.

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

A The last node

B A new empty node

C The deleted node

D The second node

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 Pointer to random node

B Pointer to next node

C Data

D Pointer to previous node

A standard doubly linked list node has data, next pointer, and previous pointer, but not a random pointer.

What is the time complexity of deleting the first node in a doubly linked list?

A O(1)

B O(n)

C O(log n)

D O(n^2)

Deleting the first node is a constant time operation because it involves updating only a few pointers.

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.

Write a simple Python function to delete the first node of a doubly linked list and return the new head.

Focus on pointer updates and edge cases.

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 in the list

D. It remains pointing to the deleted node

Solution

1. Step 1: Understand the head pointer role

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

2. Step 2: After deleting the first node

   The head 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:

   Head points to new first node after deletion [OK]

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

Common Mistakes:

• Not updating head pointer after deletion
• Setting head to null even if list has more nodes
• Leaving head pointing to deleted node

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

easy

A. head = None; head.next = None

B. head.prev = None; head = head.next

C. head = head.next; head.prev = None

D. head.next = None; head = head.prev

Solution

1. Step 1: Move head to next node

   Assign head = head.next to move head forward.

2. Step 2: Remove backward link

   Set head.prev = None to disconnect previous first node.

3. Final Answer:

   head = head.next; head.prev = None -> Option C

4. Quick Check:

   Update head then clear prev link [OK]

Hint: Update head first, then clear prev pointer [OK]

Common Mistakes:

• Setting prev before moving head
• Trying to set next of head to None incorrectly
• Setting head to None without updating

3. What will be the output linked list after deleting the first node from this doubly linked list?

Initial list: 10 <-> 20 <-> 30 <-> null

Code snippet:

head = head.next
head.prev = None

medium

A. 20 <-> 30 <-> null

B. 10 <-> 30 <-> null

C. 30 <-> null

D. null

Solution

1. Step 1: Move head to second node

   Head moves from 10 to 20.

2. Step 2: Remove backward link of new head

   Set 20.prev = None, disconnecting 10.

3. Final Answer:

   20 <-> 30 <-> null -> Option A

4. Quick Check:

   First node removed, list starts at 20 [OK]

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

Common Mistakes:

• Forgetting to set prev to None
• Incorrectly skipping nodes
• Outputting original list without change

4. Identify the error in this code snippet for deleting the first node of a doubly linked list:

if head is not None:
    head = head.next
    head.prev = None

medium

A. No error, code is correct

B. head.prev = None causes error if head.next is None

C. head = head.next should be after setting prev to None

D. Missing deletion of the old head node

Solution

1. Step 1: Check head.next for None

   If head.next is None, head becomes None after assignment.

2. Step 2: Setting head.prev causes error

   Trying to set head.prev = None when head is None causes AttributeError.

3. Final Answer:

   head.prev = None causes error if head.next is None -> Option B

4. Quick Check:

   Check for None before setting prev [OK]

Hint: Check if head is None before setting head.prev [OK]

Common Mistakes:

• Not checking if head.next is None
• Assuming list has more than one node always
• Ignoring possible AttributeError

5. Given a doubly linked list with nodes: 5 <-> 15 <-> 25 <-> 35 <-> null, after deleting the first node twice, what will be the resulting list?

hard

A. null

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

C. 35 <-> null

D. 25 <-> 35 <-> null

Solution

1. Step 1: First deletion moves head from 5 to 15

   After first deletion, list is 15 <-> 25 <-> 35 <-> null.

2. Step 2: Second deletion moves head from 15 to 25

   After second deletion, list is 25 <-> 35 <-> null.

3. Final Answer:

   25 <-> 35 <-> null -> Option D

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 each deletion
• Confusing node values or order