Pop Using Linked List Node in DSA C# - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to remove the top node from the linked list stack.

DSA C#

public int Pop()
{
    if (head == null) throw new InvalidOperationException("Stack is empty");
    int value = head.Value;
    head = head.[1];
    return value;
}

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

ANext

BPrevious

CChild

DParent

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Previous' instead of 'Next' causes errors because singly linked lists don't have previous pointers.

Using 'Child' or 'Parent' are invalid properties for linked list nodes.

The 'Next' pointer moves the head to the next node, effectively removing the top node.

2fill in blank

medium

Complete the code to check if the stack is empty before popping.

DSA C#

public int Pop()
{
    if (head == [1]) throw new InvalidOperationException("Stack is empty");
    int value = head.Value;
    head = head.Next;
    return value;
}

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

Anull

B0

Cfalse

Dtrue

Attempts:

3 left

💡 Hint

Common Mistakes

Checking head == 0 is incorrect because head is a node reference, not a number.

Using true or false is invalid for node references.

The head being null means the stack is empty.

3fill in blank

hard

Fix the error in the Pop method to correctly update the head pointer.

DSA C#

public int Pop()
{
    if (head == null) throw new InvalidOperationException("Stack is empty");
    int value = head.Value;
    head = [1];
    return value;
}

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

Anull

Bhead.Value

Chead

Dhead.Next

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning head to head.Value causes a type error.

Assigning head to null removes all nodes, not just the top.

Assigning head to head.Next moves the head to the next node, removing the top node.

4fill in blank

hard

Fill both blanks to complete the Pop method that returns the popped value and updates the head.

DSA C#

public int Pop()
{
    if (head == [1]) throw new InvalidOperationException("Stack is empty");
    int value = head.[2];
    head = head.Next;
    return value;
}

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

Anull

BValue

CNext

DPrevious

Attempts:

3 left

💡 Hint

Common Mistakes

Using head.Next in the if condition is wrong because it doesn't check if stack is empty.

Using head.Next instead of head.Value to get the value is incorrect.

Check if head is null to detect empty stack, then get the value from head.Value.

5fill in blank

hard

Fill all three blanks to implement Pop that checks empty, saves value, updates head, and returns value.

DSA C#

public int Pop()
{
    if (head == [1]) throw new InvalidOperationException("Stack is empty");
    int value = head.[2];
    head = head.[3];
    return value;
}

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

Anull

BValue

CNext

DPrevious

Attempts:

3 left

💡 Hint

Common Mistakes

Using head.Previous instead of head.Next to update head.

Getting value from head.Next instead of head.Value.

Check if head is null, get value from head.Value, then move head to head.Next.

Practice

1. What does the pop operation do in a linked list when implemented using nodes?

easy

A. Searches for a node with a specific value

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

C. Counts the total number of nodes in the linked list

D. Removes and returns the first node from the linked list

Solution

1. Step 1: Understand the pop operation

   The pop operation removes an element from a data structure. In a linked list, it usually removes the first node (head).

2. Step 2: Identify the effect on the linked list

   Removing the first node means the head pointer moves to the next node, and the removed node is returned.

3. Final Answer:

   Removes and returns the first node from the linked list -> Option D

4. Quick Check:

   Pop = Remove first node [OK]

Hint: Pop removes the first node in a linked list [OK]

Common Mistakes:

• Confusing pop with adding nodes
• Thinking pop removes the last node
• Assuming pop counts nodes

2. Which of the following is the correct syntax to pop a node from the head of a singly linked list in C#?

easy

A. head = null; return head;

B. head = head.next; return head;

C. Node temp = head; head = head.next; return temp;

D. Node temp = head.next; head = temp; return head;

Solution

1. Step 1: Save the current head node

   We store the current head in a temporary variable to return it later.

2. Step 2: Move head to the next node

   Update the head pointer to the next node in the list.

3. Step 3: Return the removed node

   Return the saved node which was the original head.

4. Final Answer:

   Node temp = head; head = head.next; return temp; -> Option C

5. Quick Check:

   Save head, move head next, return saved [OK]

Hint: Save head, move head to next, return saved node [OK]

Common Mistakes:

• Returning the new head instead of removed node
• Not saving the original head before moving
• Setting head to null incorrectly

3. Consider the following C# code for popping a node from a linked list:

Node head = new Node(10);
head.next = new Node(20);
head.next.next = new Node(30);

Node popped = Pop(ref head);
Console.WriteLine(popped.data);

What will be printed?

Assume Pop removes the first node and updates head.

medium

A. 10

B. 20

C. 30

D. NullReferenceException

Solution

1. Step 1: Understand initial linked list

   The list has nodes 10 -> 20 -> 30. Head points to node with data 10.

2. Step 2: Pop removes the first node

   Pop removes node 10 and updates head to node 20.

3. Step 3: Output the popped node's data

   The popped node's data is 10, so Console.WriteLine prints 10.

4. Final Answer:

   10 -> Option A

5. Quick Check:

   First node data popped = 10 [OK]

Hint: Pop returns first node's data before head moves [OK]

Common Mistakes:

• Printing head data after pop instead of popped node
• Confusing node values order
• Assuming pop returns null

4. The following C# method is intended to pop the first node from a linked list:

Node Pop(Node head) {
    Node temp = head;
    head = head.next;
    return temp;
}

What is the main problem with this code?

medium

A. It does not update the original head outside the method

B. It returns the wrong node

C. It causes a null pointer exception always

D. It deletes the entire linked list

Solution

1. Step 1: Analyze parameter passing

   The method receives head by value, so changes to head inside do not affect the caller's head.

2. Step 2: Understand effect on original list

   Since head is not updated outside, the original list's head remains unchanged, so pop does not remove the node effectively.

3. Final Answer:

   It does not update the original head outside the method -> Option A

4. Quick Check:

   Head passed by value, original not updated [OK]

Hint: Use ref to update head outside method [OK]

Common Mistakes:

• Assuming head changes inside method affect caller
• Thinking it returns wrong node
• Believing it causes runtime error

5. You have a singly linked list with nodes: 5 -> 15 -> 25 -> null.
You want to pop two nodes using a method that updates the head by reference.
After popping twice, what is the state of the linked list?

hard

A. 15 -> 25 -> null

B. 25 -> null

C. 5 -> 15 -> 25 -> null

D. null

Solution

1. Step 1: Initial linked list nodes

   The list starts as 5 -> 15 -> 25 -> null, head points to node with 5.

2. Step 2: Pop first node

   Pop removes node 5, head moves to node 15, list becomes 15 -> 25 -> null.

3. Step 3: Pop second node

   Pop removes node 15, head moves to node 25, list becomes 25 -> null.

4. Final Answer:

   25 -> null -> Option B

5. Quick Check:

   Two pops remove first two nodes [OK]

Hint: Each pop removes head, so two pops leave third node [OK]

Common Mistakes:

• Not updating head after pop
• Assuming popped nodes remain in list
• Confusing node order after pops