Push Using Linked List Node in DSA C# - Execution Trace

Concept Flow - Push Using Linked List Node

Create new node with data

↓

Set new node's next to current head

↓

Update head to new node

↓

Done

Push operation adds a new node at the start by linking it to the current head and then updating head to this new node.

Execution Sample

DSA C#

Node newNode = new Node(10);
newNode.next = head;
head = newNode;

This code creates a new node with value 10 and inserts it at the beginning of the linked list.

Execution Table

Step	Operation	Nodes in List	Pointer Changes	Visual State
1	Create new node with data=10	None	newNode created	null
2	Set newNode.next = head (null)	None	newNode.next -> null	10 -> null
3	Update head = newNode	One node	head -> newNode	10 -> null
4	Push complete	One node	head points to node with data 10	10 -> null

💡 Push operation ends after head points to new node.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
head	null	null	null	points to newNode	points to newNode
newNode	undefined	Node(10)	Node(10) with next=null	Node(10) with next=null	Node(10) with next=null

Key Moments - 3 Insights

Why do we set newNode.next to head before updating head?

Because newNode.next must point to the old first node to keep the list connected. See execution_table step 2 and 3 where newNode.next is set before head changes.

What if the list was empty before push?

If head is null (empty list), newNode.next becomes null, so newNode is the only node. This is shown in step 2 where newNode.next points to null.

Why does the visual state show '10 -> null' after step 2 even though head is still null?

Because newNode.next points to null, so the new node alone forms the list visually, even before head is updated in step 3.

Visual Quiz - 3 Questions

Look at the execution_table at step 2, what does newNode.next point to?

A head (which is null)

B newNode itself

C null

D previous node

💡 Hint

Check the 'Pointer Changes' column at step 2 in execution_table.

At which step does head start pointing to the new node?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Pointer Changes' column in execution_table to see when head changes.

If the list was not empty and head pointed to node with data 5, what would newNode.next point to after step 2?

A null

B node with data 5

C node with data 10

D head itself

💡 Hint

Recall that newNode.next is set to current head before head updates, see variable_tracker and concept_flow.

Concept Snapshot

Push Using Linked List Node:
- Create new node with data
- Set newNode.next to current head
- Update head to newNode
- New node becomes first element
- Works for empty or non-empty list

Full Transcript

Push operation in a linked list adds a new node at the front. First, a new node is created with the given data. Then, this new node's next pointer is set to the current head of the list, linking it to the existing nodes. Finally, the head pointer is updated to point to this new node, making it the first node in the list. This process works whether the list is empty or has existing nodes. The visual state after push shows the new node followed by the previous nodes or null if empty.

Practice

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

easy

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

B. Removes the last node from the list

C. Searches for a node with a specific value

D. Deletes the first node from the list

Solution

1. Step 1: Understand push operation in linked list

   The push operation inserts a new node at the start of the linked list.

2. Step 2: Effect of push on list structure

   The new node becomes the head, and its next pointer points to the old head.

3. Final Answer:

   Adds a new node at the beginning of the list -> Option A

4. Quick Check:

   Push means insert at head [OK]

Hint: Push always adds node at the start of the list [OK]

Common Mistakes:

• Confusing push with pop operation
• Thinking push adds at the end
• Assuming push deletes nodes

2. Which of the following is the correct syntax to create a new node and push it onto a linked list in C#?

easy

A. Node newNode = new Node(); head = newNode.next; newNode.data = data;

B. Node newNode = new Node(data); newNode.next = head; head = newNode;

C. head = new Node(data); head.next = null;

D. Node newNode = new Node(data); head = newNode.next; newNode = head;

Solution

1. Step 1: Create new node with data

   Use new Node(data) to create a node with the given data.

2. Step 2: Link new node to current head and update head

   Set newNode.next = head and then head = newNode to push at front.

3. Final Answer:

   Node newNode = new Node(data); newNode.next = head; head = newNode; -> Option B

4. Quick Check:

   New node points to old head, then head updated [OK]

Hint: New node next points to head, then head updated [OK]

Common Mistakes:

• Assigning head before setting newNode.next
• Not initializing new node with data
• Swapping head and newNode incorrectly

3. What will be the output linked list after executing this code?

Node head = null;
head = new Node(10);
head.next = new Node(20);
Node newNode = new Node(5);
newNode.next = head;
head = newNode;

Print the list from head to end.

medium

A. 10 -> 20 -> 5 -> null

B. 5 -> 20 -> 10 -> null

C. 20 -> 10 -> 5 -> null

D. 5 -> 10 -> 20 -> null

Solution

1. Step 1: Initial list creation

   Head points to node 10, which points to node 20, then null.

2. Step 2: Push new node with value 5

   New node 5 points to old head (10), then head updated to new node 5.

3. Final Answer:

   5 -> 10 -> 20 -> null -> Option D

4. Quick Check:

   New node added at front [OK]

Hint: New node points to old head, becomes new head [OK]

Common Mistakes:

• Appending new node at end instead of front
• Not updating head to new node
• Confusing node order after push

4. Identify the error in this push method for a singly linked list:

public void Push(int data) {
    Node newNode = new Node(data);
    newNode.next = null;
    head = newNode;
}

medium

A. newNode.next should point to current head before updating head

B. Node constructor is missing data parameter

C. head should be set to null before assigning newNode

D. No error, code is correct

Solution

1. Step 1: Analyze newNode.next assignment

   Setting newNode.next = null disconnects new node from existing list.

2. Step 2: Correct linking for push operation

   New node's next should point to current head to keep list intact before updating head.

3. Final Answer:

   newNode.next should point to current head before updating head -> Option A

4. Quick Check:

   Link new node to old head before head update [OK]

Hint: Link new node to old head before changing head [OK]

Common Mistakes:

• Setting newNode.next to null breaks list
• Not updating head after push
• Confusing order of assignments

5. Given a linked list with nodes 30 -> 20 -> 10 -> null, you want to push nodes 40 and 50 in that order using the push method. What will be the final list?

hard

A. 30 -> 20 -> 10 -> 50 -> 40 -> null

B. 40 -> 50 -> 30 -> 20 -> 10 -> null

C. 50 -> 40 -> 30 -> 20 -> 10 -> null

D. 30 -> 20 -> 10 -> 40 -> 50 -> null

Solution

1. Step 1: Initial list and first push

   Start with 30 -> 20 -> 10 -> null. Push 40: new head is 40, list is 40 -> 30 -> 20 -> 10 -> null.

2. Step 2: Second push with 50

   Push 50: new head is 50, list becomes 50 -> 40 -> 30 -> 20 -> 10 -> null.

3. Final Answer:

   50 -> 40 -> 30 -> 20 -> 10 -> null -> Option C

4. Quick Check:

   Push adds nodes at front in reverse order [OK]

Hint: Last pushed node becomes new head [OK]

Common Mistakes:

• Assuming push adds at end
• Mixing order of pushed nodes
• Not updating head after each push