Delete Node at Beginning in DSA C# - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to delete the first node of the linked list.

DSA C#

head = head.[1];

Drag options to blanks, or click blank then click option'

Anext

Bprev

Cvalue

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Setting head to null deletes the entire list.

Using 'prev' instead of 'next' causes errors.

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

2fill in blank

medium

Complete the code to check if the linked list is empty before deleting the first node.

DSA C#

if (head == [1]) {
    return;
}
head = head.next;

Drag options to blanks, or click blank then click option'

Ahead

Bhead.next

Cnull

Dthis

Attempts:

3 left

💡 Hint

Common Mistakes

Checking if head equals head.next is incorrect.

Using 'this' keyword is not needed here.

We check if head is null to know if the list is empty.

3fill in blank

hard

Fix the error in the code to properly delete the first node and update the tail if needed.

DSA C#

if (head == null) {
    return;
}
head = head.next;
if (head == [1]) {
    tail = null;
}

Drag options to blanks, or click blank then click option'

Atail

Bnull

Chead.next

Dhead

Attempts:

3 left

💡 Hint

Common Mistakes

Comparing head to tail instead of null.

Not updating tail when list becomes empty.

After deleting the first node, if head becomes null, the list is empty, so tail should be set to null.

4fill in blank

hard

Fill both blanks to complete the method that deletes the first node and returns its value.

DSA C#

public int DeleteFirst() {
    if (head == [1]) {
        throw new InvalidOperationException("List is empty");
    }
    int value = head.[2];
    head = head.next;
    if (head == null) {
        tail = null;
    }
    return value;
}

Drag options to blanks, or click blank then click option'

Anull

Bvalue

Cdata

Dnext

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'value' instead of 'data' to access node's value.

Not throwing exception when list is empty.

Check if head is null to detect empty list. The node's stored value is in 'data'.

5fill in blank

hard

Fill all three blanks to complete the method that deletes the first node and updates the list correctly.

DSA C#

public void DeleteFirstNode() {
    if (head == [1]) {
        return;
    }
    Node temp = head;
    head = head.[2];
    temp.[3] = null;
    if (head == null) {
        tail = null;
    }
}

Drag options to blanks, or click blank then click option'

Anull

Bnext

Dprev

Attempts:

3 left

💡 Hint

Common Mistakes

Not disconnecting the old head node.

Using 'prev' instead of 'next' to move head.

Check if head is null to avoid deleting from empty list. Move head to next node. Disconnect old head's next pointer.

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