Pop Using Linked List Node in DSA C# - Time & Space Complexity

Time Complexity: Pop Using Linked List Node

O(1)

Understanding Time Complexity

We want to understand how long it takes to remove the first item from a linked list.

How does the time needed change when the list grows bigger?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

public class Node {
    public int Value;
    public Node Next;

    public Node(int value) {
        Value = value;
        Next = null;
    }
}

public Node Pop(Node head) {
    if (head == null) return null;
    return head.Next;
}

This code removes the first node from a linked list by returning the next node as the new head.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Accessing the first node and its next pointer.
• How many times: Exactly once per pop operation, no loops or recursion.

How Execution Grows With Input

Removing the first node always takes the same steps, no matter how big the list is.

Input Size (n)	Approx. Operations
10	1
100	1
1000	1

Pattern observation: The time stays the same regardless of list size.

Final Time Complexity

Time Complexity: O(1)

This means popping the first node takes a constant amount of time no matter how large the list is.

Common Mistake

[X] Wrong: "Popping the first node takes longer if the list is bigger because we have to check all nodes."

[OK] Correct: We only change the head pointer to the next node, no need to look at other nodes.

Interview Connect

Knowing that popping from the front of a linked list is quick helps you explain efficient data structure operations clearly.

Self-Check

"What if we had to pop the last node instead? How would the time complexity change?"

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