Delete Node at Beginning in DSA C# - Cheat Sheet & Quick Revision

Recall & Review

beginner

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

It means removing the first node of the linked list, so the second node becomes the new head.

beginner

What happens to the head pointer when you delete the first node in a linked list?

The head pointer moves to the next node after the deleted node.

beginner

Why must you check if the linked list is empty before deleting the first node?

Because if the list is empty (head is null), there is no node to delete and trying to delete will cause errors.

beginner

In C#, how do you update the head to delete the first node in a singly linked list?

You set head = head.Next; this moves the head to the second node, effectively removing the first node.

beginner

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

It is O(1) because you only change the head pointer without traversing the list.

What is the first step to delete the first node in a singly linked list?

A Check if the list is empty

B Traverse to the last node

C Create a new node

D Reverse the list

Always check if the list is empty before deleting to avoid errors.

After deleting the first node, what does the head pointer point to?

A The deleted node

B The last node

C The second node

D Null always

The head pointer moves to the second node, which becomes the new first node.

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

A O(n)

B O(1)

C O(log n)

D O(n^2)

Deleting the first node is done in constant time O(1).

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

A List becomes circular

B Head points to the same node

C Head points to a new node

D Head points to null

After deleting the only node, the list becomes empty and head points to null.

Which C# statement deletes the first node in a singly linked list?

A head = head.Next;

B head.Next = head;

C head = null;

D head = new Node();

Setting head to head.Next moves the head to the second node, deleting the first.

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

Think about what happens to the head pointer and the list structure.

Describe the time complexity and why deleting the first node in a linked list is efficient.

Focus on how many steps are needed to delete the first node.

Practice

1. What happens when you delete the first node in a singly linked list?

easy

A. The last node is deleted instead of the first

B. Nothing happens because the first node cannot be deleted

C. The entire list is deleted

D. The head pointer moves to the second node, removing the first node from the list

Solution

1. Step 1: Understand the head pointer role

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

2. Step 2: Update head to next node

   Deleting the first node means moving the head pointer to the second node, effectively removing the first node.

3. Final Answer:

   The head pointer moves to the second node, removing the first node from the list -> Option D

4. Quick Check:

   Delete first node = move head to next [OK]

Hint: Delete first node by moving head to next node [OK]

Common Mistakes:

• Trying to delete the last node instead
• Not updating the head pointer
• Deleting the entire list accidentally

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

easy

A. head = null.next;

B. head.next = head;

C. head = head.next;

D. head.next = null;

Solution

1. Step 1: Identify the head pointer update

   To delete the first node, assign head to head.next to move the head pointer forward.

2. Step 2: Check syntax correctness

   head = head.next; correctly updates the head pointer to the second node.

3. Final Answer:

   head = head.next; -> Option C

4. Quick Check:

   Correct syntax to delete first node = head = head.next; [OK]

Hint: Assign head to head.next to delete first node [OK]

Common Mistakes:

• Reversing the assignment order
• Using null.next which is invalid
• Setting head.next to null instead of head

3. Consider the following code snippet for deleting the first node in a linked list:

Node head = new Node(1);
head.next = new Node(2);
head.next.next = new Node(3);
head = head.next;

What is the state of the list after deletion?

medium

A. 2 -> 3 -> null

B. null

C. 3 -> null

D. 1 -> 2 -> 3 -> null

Solution

1. Step 1: Initial list creation

   The list starts as 1 -> 2 -> 3 -> null with head at node 1.

2. Step 2: Delete first node by moving head

   head = head.next moves head to node 2, so the list becomes 2 -> 3 -> null.

3. Final Answer:

   2 -> 3 -> null -> Option A

4. Quick Check:

   After deletion, list starts at second node [OK]

Hint: After deletion, list starts from second node [OK]

Common Mistakes:

• Forgetting to update head pointer
• Assuming first node still exists
• Confusing node values after deletion

4. The following code attempts to delete the first node of a linked list but causes a runtime error:

Node head = null;
head = head.next;

What is the error and how to fix it?

medium

A. SyntaxError; fix by adding semicolon

B. NullReferenceException; fix by checking if head is not null before deleting

C. IndexOutOfRangeException; fix by resizing list

D. No error; code works fine

Solution

1. Step 1: Identify cause of error

   head is null, so accessing head.next causes NullReferenceException.

2. Step 2: Fix by adding null check

   Check if head != null before assigning head = head.next to avoid error.

3. Final Answer:

   NullReferenceException; fix by checking if head is not null before deleting -> Option B

4. Quick Check:

   Null head causes error; check before delete [OK]

Hint: Always check head != null before deleting first node [OK]

Common Mistakes:

• Ignoring null checks causing runtime errors
• Assuming list always has nodes
• Confusing syntax errors with runtime errors

5. You have a singly linked list with nodes: 5 -> 10 -> 15 -> 20 -> null. You delete the first node twice using head = head.next; two times. What is the resulting list?

hard

A. 15 -> 20 -> null

B. 10 -> 15 -> 20 -> null

C. 20 -> null

D. null

Solution

1. Step 1: Initial list and first deletion

   List starts as 5 -> 10 -> 15 -> 20 -> null. After first deletion, head moves to 10, list is 10 -> 15 -> 20 -> null.

2. Step 2: Second deletion moves head again

   After second deletion, head moves to 15, list is 15 -> 20 -> null.

3. Final Answer:

   15 -> 20 -> null -> Option A

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
• Confusing node values after multiple deletions
• Assuming list becomes empty too early