Delete Node at End in DSA C# - 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

In a singly linked list, why do we need to traverse the list to delete the last node?

Because we only have a reference to the head and each node points to the next, we must find the second last node to update its next pointer to null.

beginner

What happens if you delete the last node in a linked list with only one node?

The list becomes empty, so the head pointer should be set to null.

intermediate

Show the key steps to delete the last node in a singly linked list.

1. Check if list is empty (head is null). 2. If only one node, set head to null. 3. Otherwise, traverse to second last node. 4. Set second last node's next to null.

beginner

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

O(n), because you must 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 next pointer of the second last node

B The head pointer

C The next pointer of the last node

D The previous pointer of the last node

You update the next pointer of the second last node to null to remove the last node.

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

A The head points to the deleted node

B The head becomes null

C The list still contains one node

D The list becomes circular

Deleting the only node makes the list empty, so head is set to null.

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.

Which of these is NOT a step in deleting the last node in a singly linked list?

A Traverse to the second last node

B Set the second last node's next to null

C Update the head to the last node

D Check if the list is empty

You do not update the head to the last node; you remove the last node.

Why can't you delete the last node in O(1) time in a singly linked list?

A Because the last node has no next pointer

B Because the list is circular

C Because the head pointer is always null

D Because you need to find the previous node first

You need to find the node before the last to update its next pointer, which requires traversal.

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

Think about what happens when the list has zero, one, or many nodes.

Why is deleting the last node in a singly linked list slower than deleting the first node?

Consider how pointers are arranged in singly linked lists.

Practice

1. What does the operation Delete Node at End do in a singly linked list?

easy

A. Adds a new node at the end of the list

B. Removes the first node from the list

C. Removes the last node from the list

D. Adds a new node at the beginning of the list

Solution

1. Step 1: Understand the operation meaning

   Deleting a node at the end means removing the last node in the linked list.

2. Step 2: Compare options with operation

   Only Removes the last node from the list matches removing the last node; others add or remove nodes elsewhere.

3. Final Answer:

   Removes the last node from the 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
• Assuming it deletes a middle node

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

easy

A. Create a new node and assign it to last node

B. Set head to null directly

C. Set last node's next to head

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

Solution

1. Step 1: Identify correct deletion method

   To delete last node, we must reach second last node and set its next pointer to null.

2. Step 2: Check options for this logic

   Traverse to second last node, set its next to null matches this logic; others do unrelated actions.

3. Final Answer:

   Traverse to second last node, 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 [OK]

Common Mistakes:

• Setting head to null deletes whole list
• Trying to set last node's next to head causes cycle
• Creating new node does not delete existing node

3. Given the linked list: 10 -> 20 -> 30 -> 40 -> null, what will be the list after deleting the node at the end?

medium

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

B. 10 -> 20 -> 30 -> null

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

D. 10 -> 20 -> null

Solution

1. Step 1: Identify last node in list

   The last node is 40 in the list 10 -> 20 -> 30 -> 40 -> null.

2. Step 2: Remove last node and update list

   Deleting last node removes 40, so list becomes 10 -> 20 -> 30 -> null.

3. Final Answer:

   10 -> 20 -> 30 -> null -> Option B

4. Quick Check:

   Last node 40 removed = 10->20->30->null [OK]

Hint: Remove last value to get new list [OK]

Common Mistakes:

• Not removing last node, list unchanged
• Removing first node instead
• Removing more than one node accidentally

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

Node current = head;
while(current.next.next != null) {
    current = current.next;
}
current.next = null;

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

medium

A. NullReferenceException because current.next.next is invalid

B. No error, code works fine

C. Infinite loop because condition never false

D. Deletes the head node incorrectly

Solution

1. Step 1: Analyze condition for single node list

   If list has one node, current.next is null, so current.next.next causes error.

2. Step 2: Identify exception type

   Accessing next of null causes NullReferenceException in C#.

3. Final Answer:

   NullReferenceException because current.next.next is invalid -> Option A

4. Quick Check:

   Single node list causes null.next access error [OK]

Hint: Check for null before accessing next.next [OK]

Common Mistakes:

• Ignoring null check for single node
• Assuming loop runs safely always
• Thinking it deletes head node

5. You have a singly linked list with nodes: 5 -> 15 -> 25 -> 35 -> 45 -> null. Write a method to delete the last node and return the updated list. Which of the following code snippets correctly implements this?

hard

A. if (head == null) return null;
if (head.next == null) return null;
Node current = head;
while (current.next.next != null) {
current = current.next;
}
current.next = null;
return head;

B. if (head == null) return null;
Node current = head;
while (current.next != null) {
current = current.next;
}
current = null;
return head;

C. if (head == null) return null;
Node current = head;
while (current.next.next != null) {
current = current.next;
}
current.next = current;
return head;

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

Solution

1. Step 1: Handle empty and single node cases

   If list is empty or has one node, deleting last node returns null (empty list).

2. Step 2: Traverse to second last node and delete last

   Loop until current.next.next is null, then set current.next to null to remove last node.

3. Step 3: Return updated head

   Return head pointer to updated list.

4. Final Answer:

   Code snippet D correctly deletes last node and returns updated list -> Option A

5. Quick Check:

   Check null, traverse to second last, set next null [OK]

Hint: Check list size, then remove last by second last next = null [OK]

Common Mistakes:

• Setting current to null does not remove last node
• Assigning current.next to current creates cycle
• Deleting head instead of last node