Insert at Beginning Head Insert in DSA C - Execution Trace

Concept Flow - Insert at Beginning Head Insert

Create new node

↓

Set new_node->next = head

↓

Update head = new_node

↓

New node is now the first node in list

This flow shows how a new node is created and placed at the start of the linked list by adjusting pointers.

Execution Sample

DSA C

struct Node {
    int data;
    struct Node* next;
};

void insertAtBeginning(struct Node** head, int val) {
    struct Node* new_node = malloc(sizeof(struct Node));
    new_node->data = val;
    new_node->next = *head;
    *head = new_node;
}

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

Execution Table

Step	Operation	Nodes in List	Pointer Changes	Visual State
1	Start with empty list	∅	head = ∅	List is empty: head → ∅
2	Create new node with data=10	Node(10)	new_node created, next unassigned	new_node: [data:10 | next: ?]
3	Set new_node->next = head (∅)	Node(10)	new_node->next → ∅	new_node: [data:10 | next: ∅]
4	Update head = new_node	Node(10)	head → new_node	head → [data:10 | next: ∅]
5	Insert new node with data=20 at beginning	Node(20) → Node(10)	new_node created, new_node->next = head, head updated	head → [data:20 | next: ] → [data:10 | next: ∅]
6	Insert new node with data=30 at beginning	Node(30) → Node(20) → Node(10)	new_node created, new_node->next = head, head updated	head → [data:30 | next: ] → [data:20 | next: ] → [data:10 | next: ∅]
7	End of insertions	Node(30) → Node(20) → Node(10)	No pointer changes	Final list: head → 30 → 20 → 10 → ∅

💡 All insertions done, head points to newest node, list built from front

Variable Tracker

Variable	Start	After Step 2	After Step 4	After Step 5	After Step 6	Final
head	∅	∅	[data:10 | next: ∅]	[data:20 | next: ]	[data:30 | next: ]	[data:30 | next: ]
new_node	∅	[data:10 | next: ?]	[data:10 | next: ∅]	[data:20 | next: ]	[data:30 | next: ]	∅ (no longer needed)

Key Moments - 3 Insights

Why do we set new_node->next = head before updating head?

Because we want the new node to point to the current first node before head changes. See execution_table step 3 and 4 where new_node->next is set to old head, then head is updated to new_node.

What happens if the list is empty when inserting?

The new node's next points to null (∅), and head points to the new node. This is shown in steps 1 to 4 where head starts as ∅ and after insertion points to the new node.

How does the list grow after multiple insertions at beginning?

Each new node points to the previous head, making it the new first node. The list visually grows from the front as shown in steps 5 and 6.

Visual Quiz - 3 Questions

Look at the execution_table at step 4, what does head point to?

A The old head before insertion

B Null (∅)

C The new node with data 10

D The new node with data 20

💡 Hint

Check the Pointer Changes and Visual State columns at step 4 in execution_table

At which step does the list first contain two nodes?

A Step 5

B Step 4

C Step 3

D Step 6

💡 Hint

Look at the Nodes in List column to see when two nodes appear

If we skipped setting new_node->next = head, what would happen?

A The list would be empty

B The new node would not link to the rest of the list

C The head pointer would not change

D The new node would point to itself

💡 Hint

Refer to the key moment about why new_node->next is set before updating head

Concept Snapshot

Insert at Beginning (Head Insert):
- Create new node
- Set new_node->next = head
- Update head = new_node
- New node becomes first in list
- List grows by adding nodes at front

Full Transcript

This concept shows how to insert a new node at the beginning of a singly linked list. First, a new node is created with the given data. Then, the new node's next pointer is set to the current head of the list. Finally, the head pointer is updated to point to the new node. This makes the new node the first node in the list. Repeating this process adds nodes to the front, growing the list from the beginning.