Delete Node at End in DSA Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does 'Delete Node at End' mean in a linked list?

It means removing the last node from a linked list, so the second last node becomes the new end.

beginner

Why do we need to find the second last node when deleting the last node in a singly linked list?

Because the second last node's next pointer must be set to None to remove the last node.

beginner

What happens if you try to delete the last node in an empty linked list?

Nothing happens because the list is empty; there is no node to delete.

beginner

In a singly linked list with one node, what happens when you delete the last node?

The list becomes empty because the only node is removed.

intermediate

What is the time complexity of deleting the last node in a singly linked list?

O(n), because you need to traverse the list to find the second last node.

What pointer do you update when deleting the last node in a singly linked list?

A The previous pointer of the last node

B The next pointer of the second last node

C The last node's next pointer

D The head pointer

You update the next pointer of the second last node to None to remove the last node.

What is the new last node after deleting the last node?

A The node before the deleted node

B The head node

C The deleted node itself

D The node after the deleted node

The node before the deleted node becomes the new last node.

If a linked list has only one node, what happens after deleting the last node?

A The list is unchanged

B The list has one node left

C The list has two nodes

D The list becomes empty

Deleting the only node makes the list empty.

What is the time complexity to delete the last node in a singly linked list?

A O(n)

B O(1)

C O(log n)

D O(n^2)

You must traverse the list to find the second last node, so it takes O(n) time.

What happens if you try to delete the last node from an empty linked list?

A Error occurs

B A new node is added

C Nothing happens

D The list becomes full

Since the list is empty, there is no node to delete, so nothing happens.

Explain step-by-step how to delete the last node in a singly linked list.

Think about what happens when the list is empty or has one node.

Describe the changes in pointers when deleting the last node in a singly linked list.

Focus on the next pointer of the node before the last.

Practice

1. What is the main purpose of the Delete Node at End operation in a singly linked list?

easy

A. Remove the first node from the linked list

B. Add a new node at the end of the linked list

C. Remove the last node from the linked list

D. Reverse the linked list

Solution

1. Step 1: Understand the operation

   The operation aims to remove a node from the end of the singly linked list.

2. Step 2: Identify the correct description

   Removing the last node means deleting the tail node, not adding or reversing nodes.

3. Final Answer:

   Remove the last node from the linked list -> Option C

4. Quick Check:

   Delete Node at End = Remove last node [OK]

Hint: Deleting at end means removing the last node [OK]

Common Mistakes:

• Confusing delete at end with delete at beginning
• Thinking it adds a node instead of removing
• Mixing up with reversing the list

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

easy

A. Set last node's next to head

B. Set head = null directly

C. Create a new node and assign it to head

D. Traverse to second last node and set its next to null

Solution

1. Step 1: Understand deletion at end

   To delete the last node, we must find the second last node and make its next pointer null.

2. Step 2: Check each option

   Setting head = null deletes the whole list, not just last node. Setting last node's next to head creates a cycle. Creating a new node and assigning to head adds a node, not deletes.

3. Final Answer:

   Traverse to second last node and set its next to null -> Option D

4. Quick Check:

   Delete last node = second last node next = null [OK]

Hint: Set second last node's next to null to delete last node [OK]

Common Mistakes:

• Setting head to null deletes entire list
• Creating cycles by wrong pointer assignments
• Adding nodes instead of deleting

3. What will be the output linked list after deleting the last node from the list: 1 -> 2 -> 3 -> 4 -> null?

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 in the list is the node with value 4.

2. Step 2: Delete the last node

   Removing node 4 means the new last node is 3, so the list ends at 3 with next as null.

3. Final Answer:

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

4. Quick Check:

   Last node 4 removed = 1 -> 2 -> 3 [OK]

Hint: Last node removed means list ends one node earlier [OK]

Common Mistakes:

• Not updating second last node's next to null
• Removing wrong node (like first node)
• Confusing output with original list

4. Consider this code snippet to delete the last node of a singly linked list:

def delete_end(head):
    if head is None:
        return None
    if head.next is None:
        return None
    temp = head
    while temp.next is not None:
        temp = temp.next
    temp.next = None
    return head

What is the error in this code?

medium

A. The loop never stops because condition is wrong

B. temp.next is null check misses second last node

C. It deletes the first node instead of last

D. No error, code works correctly

Solution

1. Step 1: Analyze the loop condition

   The loop runs while temp.next is not null, so temp moves to the last node.

2. Step 2: Identify pointer update mistake

   After loop, temp points to last node, but to delete last node, we need second last node to set its next to null. Here, temp.next = null sets last node's next to null, which is already null, so no deletion happens.

3. Final Answer:

   temp.next is null check misses second last node -> Option B

4. Quick Check:

   Must stop at second last node to delete last [OK]

Hint: Stop at second last node, not last, to delete end node [OK]

Common Mistakes:

• Stopping loop at last node instead of second last
• Not handling single node list correctly
• Setting wrong pointers causing no deletion

5. Given a singly linked list with nodes 10 -> 20 -> 30 -> 40 -> 50 -> null, write the steps to delete the last node and then add a new node with value 60 at the end. What will be the final list?

hard

A. 10 -> 20 -> 30 -> 40 -> 60 -> null

B. 10 -> 20 -> 30 -> 40 -> 50 -> 60 -> null

C. 10 -> 20 -> 30 -> 40 -> null

D. 10 -> 20 -> 30 -> 40 -> 50 -> null

Solution

1. Step 1: Delete the last node (50)

   Traverse to second last node (40) and set its next to null, removing 50.

2. Step 2: Add new node with value 60 at end

   Traverse to last node (40) and set its next to new node with value 60, new node's next is null.

3. Final Answer:

   10 -> 20 -> 30 -> 40 -> 60 -> null -> Option A

4. Quick Check:

   Delete last 50, add 60 at end = 10->20->30->40->60 [OK]

Hint: Delete last node, then link new node at end [OK]

Common Mistakes:

• Not updating second last node's next to null before adding
• Appending new node without deleting last node
• Confusing node values or order