Delete from End 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 moving forward and backward through the list.

beginner

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

The last node is removed, the second last node's next pointer is set to null, and the list's tail pointer is updated to the second last node.

intermediate

Why do we need to update the previous node's next pointer when deleting from the end of a doubly linked list?

Because the new last node should not point to the deleted node, so its next pointer must be set to null to mark the end of the list.

intermediate

What is the time complexity of deleting the last node in a doubly linked list if we have a tail pointer?

O(1) because we can directly access the last node using the tail pointer without traversing the list.

beginner

Show the steps to delete the last node in a doubly linked list with nodes: 1 <-> 2 <-> 3 <-> null

1. Identify the last node (3).
2. Set the next pointer of node 2 to null.
3. Update the tail pointer to node 2.
Resulting list: 1 <-> 2 <-> null

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

A The previous node's next pointer

B The next node's previous pointer

C The head pointer

D The data pointer

The previous node's next pointer must be set to null to mark the new end of the list.

If a doubly linked list has only one node and you delete from the end, what happens to the list?

A The head pointer points to the deleted node

B The list still has one node

C The tail pointer points to the deleted node

D The list becomes empty

Deleting the only node makes the list empty, so head and tail pointers become null.

What is the time complexity of deleting the last node if no tail pointer is maintained?

A O(log n)

B O(1)

C O(n)

D O(n^2)

Without a tail pointer, you must traverse the list to find the second last node, which takes O(n) time.

Which pointer in the last node of a doubly linked list is always null?

A Previous pointer

B Next pointer

C Data pointer

D Head pointer

The next pointer of the last node is always null because there is no node after it.

After deleting the last node, what should the tail pointer point to?

A The new last node

B Null

C The deleted node

D The head node

The tail pointer should update to the new last node to keep the list consistent.

Explain the steps to delete the last node from a doubly linked list and update pointers correctly.

Think about what happens to the node before the last and the tail pointer.

Describe how the presence of a tail pointer affects the efficiency of deleting from the end of a doubly linked list.

Compare with singly linked list deletion from end.

Practice

1. What happens when you delete a node from the end of a doubly linked list that has multiple nodes?

easy

A. The last node is removed and the second last node becomes the new tail.

B. The first node is removed and the second node becomes the new head.

C. All nodes are deleted and the list becomes empty.

D. The middle node is removed and the list remains unchanged at the ends.

Solution

1. Step 1: Identify the last node and update pointers

   In a doubly linked list, the last node is the tail node which has its next pointer as null. Deleting the last node means updating the tail pointer to the previous node and setting its next pointer to null. The second last node becomes the new tail.

2. Final Answer:

   The last node is removed and the second last node becomes the new tail. -> Option A

3. Quick Check:

   Delete end node = tail moves back [OK]

Hint: Last node removed, tail moves to previous node [OK]

Common Mistakes:

• Removing the head instead of the tail
• Not updating the tail pointer
• Leaving the next pointer of new tail non-null

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

easy

A. head = head.prev; head.next = None

B. tail = tail.next; tail.prev = None

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

D. tail = tail.prev; tail.next = None

Solution

1. Step 1: Move tail pointer and update next

   To delete the last node, move tail to tail.prev (second last node), then set tail.next to None to mark the end of the list.

2. Final Answer:

   tail = tail.prev; tail.next = None -> Option D

3. Quick Check:

   Correct tail update syntax = tail = tail.prev; tail.next = None [OK]

Hint: Tail moves back, next pointer set to None [OK]

Common Mistakes:

• Moving tail forward instead of backward
• Setting prev pointer instead of next
• Changing head pointer instead of tail

3. What will be the output after deleting the last node from the doubly linked list shown below?

Initial list: 10 <-> 20 <-> 30 <-> 40 <-> None
Operation: Delete from end

What is the resulting list?

medium

A. 10 <-> 20 <-> 30 <-> 40 <-> None

B. 10 <-> 20 <-> 30 <-> None

C. 20 <-> 30 <-> 40 <-> None

D. 10 <-> 20 <-> None

Solution

1. Step 1: Identify last node and update

   The last node is 40, which is removed by setting node 30's next pointer to None, making it the new tail. Result: 10 <-> 20 <-> 30 <-> None.

2. Final Answer:

   10 <-> 20 <-> 30 <-> None -> Option B

3. Quick Check:

   Last node 40 removed, tail is 30 [OK]

Hint: Remove last node, update previous node's next to None [OK]

Common Mistakes:

• Not updating the next pointer of new tail
• Removing the wrong node (head or middle)
• Leaving dangling pointers causing errors

4. Consider the following code snippet to delete the last node from a doubly linked list:

if tail is None:
    print('List is empty')
else:
    tail = tail.prev
    tail.next = None

What error can occur when this code runs on a list with only one node?

medium

A. AttributeError because tail.prev is None and tail.next is accessed

B. No error, code works fine for single node list

C. SyntaxError due to missing colon

D. IndexError because tail is out of range

Solution

1. Step 1: Trace execution for single node

   For one node, tail.prev is None, so tail = None, then tail.next = None accesses None.next causing AttributeError.

2. Final Answer:

   AttributeError because tail.prev is None and tail.next is accessed -> Option A

3. Quick Check:

   Single node delete needs extra check [OK]

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

Common Mistakes:

• Not handling single node list case
• Assuming tail.prev always exists
• Ignoring NoneType attribute errors

5. You have a doubly linked list with nodes: 5 <-> 15 <-> 25 <-> 35 <-> 45 <-> None. You delete the last node twice in a row. What will be the final list?

hard

A. 5 <-> 15 <-> 25 <-> 35 <-> 45 <-> None

B. 5 <-> 15 <-> 25 <-> 35 <-> None

C. 5 <-> 15 <-> 25 <-> None

D. 15 <-> 25 <-> 35 <-> None

Solution

1. Step 1: Perform two deletions

   Initial: 5 <-> 15 <-> 25 <-> 35 <-> 45. First delete removes 45 (tail=35). Second delete removes 35 (tail=25). Final: 5 <-> 15 <-> 25 <-> None.

2. Final Answer:

   5 <-> 15 <-> 25 <-> None -> Option C

3. Quick Check:

   Two deletes remove last two nodes [OK]

Hint: Each delete removes one tail node [OK]

Common Mistakes:

• Deleting only one node instead of two
• Not updating tail pointer after each delete
• Confusing head and tail nodes