Insert at End Tail Insert in DSA C - Execution Trace

Concept Flow - Insert at End Tail Insert

Create new node with data

↓

Is list empty?

↓

Set head = new

↓

Traverse until current.next == NULL

↓

Set current.next = new node

↓

Done

Create a new node, check if list is empty, if yes set head to new node; else traverse to last node and link new node at end.

Execution Sample

DSA C

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

void insertAtEnd(struct Node** head, int val) {
  // Insert val at list end
  struct Node* new_node = (struct Node*)malloc(sizeof(struct Node));
  new_node->data = val;
  new_node->next = NULL;

  if (*head == NULL) {
    *head = new_node;
    return;
  }

  struct Node* current = *head;
  while (current->next != NULL) {
    current = current->next;
  }
  current->next = new_node;
}

Insert a new node with given value at the end of a singly linked list.

Execution Table

Step	Operation	Nodes in List	Pointer Changes	Visual State
1	Create new node with data=10	∅	new_node created with data=10, next=∅	∅
2	Check if head == NULL	∅	head == NULL (list empty)	∅
3	Set head = new_node	10	head → new_node	┌────────┐ │ data:10│ │ next:∅ │ └────────┘ head
4	Create new node with data=20	10	new_node created with data=20, next=∅	┌────────┐ ┌────────┐ │ data:10│ │ data:20│ │ next:∅ │ │ next:∅ │ └────────┘ └────────┘ head
5	Check if head == NULL	10	head != NULL (list not empty)	┌────────┐ ┌────────┐ │ data:10│ │ data:20│ │ next:∅ │ │ next:∅ │ └────────┘ └────────┘ head
6	Traverse to last node (current = head)	10	current → node with data=10	┌────────┐ ┌────────┐ │ data:10│ │ data:20│ │ next:∅ │ │ next:∅ │ └────────┘ └────────┘ head new_node
7	Check current.next == NULL	10	current.next == NULL (true)	┌────────┐ ┌────────┐ │ data:10│ │ data:20│ │ next:∅ │ │ next:∅ │ └────────┘ └────────┘ head new_node
8	Set current.next = new_node	10 -> 20	node 10.next → new_node	┌────────┐ ┌────────┐ │ data:10│───→│ data:20│ │ next:──┼ │ next:∅ │ └────────┘ └────────┘ head ✓ tail
9	Done	10 -> 20	No pointer changes	┌────────┐ ┌────────┐ │ data:10│───→│ data:20│ │ next:──┼ │ next:∅ │ └────────┘ └────────┘ head tail

💡 Traversal ends when current.next is NULL; new node linked as last node.

Variable Tracker

Variable	Start	After Step 1	After Step 3	After Step 4	After Step 6	After Step 8	Final
head	NULL	NULL	points to node 10	points to node 10	points to node 10	points to node 10	points to node 10
new_node	NULL	node 10	node 10	node 20	node 20	node 20	node 20
current	NULL	NULL	NULL	NULL	node 10	node 10	node 10

Key Moments - 3 Insights

Why do we check if head == NULL before inserting?

Because if the list is empty (head == NULL), we must set head to the new node directly (see step 2 and 3 in execution_table). Otherwise, we traverse the list.

Why do we stop traversal when current.next == NULL, not current == NULL?

Because current.next == NULL means current is the last node. We want to link the new node after current. If we checked current == NULL, we would go past the last node (see step 7).

What pointer changes when we insert the new node at the end?

The last node's next pointer changes from NULL to point to the new node (see step 8). The new node's next remains NULL.

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what does head point to?

A The new node with data 10

B NULL

C The new node with data 20

D The last node in the list

💡 Hint

Check the 'Pointer Changes' and 'Visual State' columns at step 3.

At which step does the traversal stop before inserting the new node?

A Step 5

B Step 6

C Step 7

D Step 8

💡 Hint

Look for the step where current.next == NULL is true in the execution_table.

If the list was empty, which step sets the head pointer?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

See where head is assigned to new_node in the execution_table.

Concept Snapshot

Insert at End (Tail Insert) in Linked List:
- Create new node with data and next = NULL
- If list empty (head == NULL), set head = new node
- Else traverse list until last node (current.next == NULL)
- Set last node's next = new node
- New node becomes new tail with next = NULL

Full Transcript

This concept shows how to insert a new node at the end of a singly linked list. First, a new node is created with the given data and its next pointer set to NULL. Then, we check if the list is empty by seeing if head is NULL. If empty, we set head to the new node. If not empty, we start from head and traverse nodes until we find the last node whose next pointer is NULL. We then set that last node's next pointer to the new node, effectively adding it at the end. The new node's next remains NULL, marking it as the new tail. This process ensures the list grows by adding nodes at the end.