Delete from End of Doubly Linked List in DSA Typescript - 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 if the list becomes empty, the head pointer is also set to null.

intermediate

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

Because each node points back to the previous node, when the last node is removed, the new last node's next pointer must be set to null, and its previous pointer remains unchanged to maintain list integrity.

intermediate

Show the TypeScript code snippet to delete the last node from a doubly linked list.

```typescript
function deleteFromEnd(head: Node | null): Node | null {
  if (!head) return null;
  if (!head.next) return null; // only one node
  let current = head;
  while (current.next) {
    current = current.next;
  }
  if (current.prev) {
    current.prev.next = null;
  }
  return head;
}
```

intermediate

What is the time complexity of deleting the last node in a doubly linked list if we do not keep a tail pointer?

O(n), because we need to traverse from the head to the last node to delete it.

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

A The head pointer only

B The next node's previous pointer

C The previous node's next pointer

D No pointers need updating

The previous node's next pointer must be set to null to remove the last node.

If the doubly linked list has only one node and we delete from the end, what is the new head?

A The same node

B The previous node

C The next node

D Null

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

What is the time complexity of deleting the last node if the list has a tail pointer?

A O(n)

B O(1)

C O(log n)

D O(n^2)

With a tail pointer, deletion from the end is direct and takes constant time.

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

A Pointer to head node

B Pointer to next node

C Pointer to previous node

D Data

Nodes do not have a pointer to the head node.

What happens if you forget to update the previous node's next pointer when deleting the last node?

A The last node remains linked, causing errors

B The list becomes circular

C The list becomes empty

D Nothing happens

The last node would still be linked, causing incorrect list structure.

Explain step-by-step how to delete the last node from a doubly linked list without a tail pointer.

Think about how to find the last node and what pointers to change.

Describe the difference in deleting the last node when a tail pointer is present versus when it is not.

Focus on how the tail pointer changes the process.

Practice

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

easy

A. It becomes null immediately

B. It moves to the previous node of the deleted node

C. It points to the head node

D. It remains unchanged

Solution

1. Step 1: Understand the tail pointer role

   The tail pointer always points to the last node in the list.

2. Step 2: Effect of deleting the last node

   When the last node is deleted, the tail must move to the previous node to remain the last node.

3. Final Answer:

   It moves to the previous node of the deleted node -> Option B

4. Quick Check:

   Tail updates to previous node after deletion [OK]

Hint: Tail always points to last node, so move it back after deletion [OK]

Common Mistakes:

• Setting tail to null without checking if list is empty
• Not updating tail pointer after deletion
• Confusing head and tail pointers
• Assuming tail points to head after deletion

2. Which of the following is the correct TypeScript syntax to delete the last node from a doubly linked list assuming tail is not null?

easy

A. tail = tail.prev; tail.next = null;

B. tail = tail.next; tail.prev = null;

C. tail = null; head = null;

D. tail.prev = tail; tail.next = null;

Solution

1. Step 1: Move tail pointer to previous node

   To delete the last node, tail must point to its previous node: tail = tail.prev;

2. Step 2: Set new tail's next to null

   Since tail is now last node, its next must be null: tail.next = null;

3. Final Answer:

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

4. Quick Check:

   Correct syntax moves tail back and nullifies next [OK]

Hint: Tail moves back, then tail.next set to null [OK]

Common Mistakes:

• Moving tail forward instead of backward
• Not setting tail.next to null
• Setting tail to null without updating links
• Confusing prev and next pointers

3. Given the doubly linked list: 1 <-> 2 <-> 3 <-> 4 <-> null, what is the list after deleting the last node?

medium

A. 1 <-> 2 <-> 3 <-> null

B. 1 <-> 2 <-> 3 <-> 4 <-> null

C. 2 <-> 3 <-> 4 <-> null

D. 1 <-> 2 <-> null

Solution

1. Step 1: Identify the last node

   The last node is 4, pointed by tail.

2. Step 2: Delete last node and update tail

   Remove node 4, update tail to node 3, set tail.next to null.

3. Final Answer:

   1 <-> 2 <-> 3 <-> null -> Option A

4. Quick Check:

   Last node 4 removed, tail now 3 [OK]

Hint: Remove last node, tail moves to previous node [OK]

Common Mistakes:

• Not updating tail pointer
• Leaving deleted node linked
• Removing wrong node
• Confusing head and tail updates

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

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

What error can occur here?

medium

A. No error, code is correct

B. Syntax error due to missing semicolon

C. Runtime error if list has only one node

D. Tail pointer moves forward incorrectly

Solution

1. Step 1: Analyze single-node list case

   If list has one node, tail.prev is null, so tail becomes null.

2. Step 2: Accessing tail.next causes error

   After tail = tail.prev (null), next line tries tail.next = null, causing runtime error (cannot access property of null).

3. Final Answer:

   Runtime error if list has only one node -> Option C

4. Quick Check:

   Null tail causes runtime error on next access [OK]

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

Common Mistakes:

• Not handling single-node list edge case
• Assuming tail is never null after update
• Ignoring runtime null pointer errors
• Confusing syntax errors with runtime errors

5. You have a doubly linked list with nodes: 10 <-> 20 <-> 30. After deleting the last node twice, what is the final list?

hard

A. 10 <-> 20 <-> null

B. 20 <-> null

C. null

D. 10 <-> null

Solution

1. Step 1: Delete last node first time

   Last node 30 is removed, tail moves to 20, list becomes 10 <-> 20 <-> null.

2. Step 2: Delete last node second time

   Last node 20 is removed, tail moves to 10, list becomes 10 <-> null.

3. Step 3: Verify final list

   After two deletions, list is 10 <-> null.

4. Final Answer:

   10 <-> null -> Option D

5. Quick Check:

   Two deletions remove 30 and 20, leaving 10 [OK]

Hint: Delete last node twice removes last two nodes [OK]

Common Mistakes:

• Deleting only one node instead of two
• Confusing node values after deletion
• Not updating tail pointer correctly
• Assuming list becomes empty too early