Enqueue Using Linked List in DSA C - Execution Trace

Concept Flow - Enqueue Using Linked List

Create new node with data

↓

Is queue empty?

Yes→Set head and tail to new node

No↓

Set tail.next to new node

↓

Update tail to new node

↓

Done

This flow shows how a new node is created and added at the end of the linked list representing the queue.

Execution Sample

DSA C

void enqueue(int data) {
  Node* newNode = malloc(sizeof(Node));
  newNode->data = data;
  newNode->next = NULL;
  if (tail == NULL) {
    head = tail = newNode;
  } else {
    tail->next = newNode;
    tail = newNode;
  }
}

This code adds a new element at the end of the queue using a 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	Check if queue empty (tail == NULL)	None	tail is NULL	null
3	Set head and tail to newNode	10	head -> newNode, tail -> newNode	10 -> null
4	Create new node with data=20	10	newNode created	10 -> null
5	Check if queue empty (tail == NULL)	10	tail is not NULL	10 -> null
6	Set tail->next to newNode	10, 20	tail->next -> newNode	10 -> 20 -> null
7	Update tail to newNode	10, 20	tail -> newNode	10 -> 20 -> null
8	Create new node with data=30	10, 20	newNode created	10 -> 20 -> null
9	Check if queue empty (tail == NULL)	10, 20	tail is not NULL	10 -> 20 -> null
10	Set tail->next to newNode	10, 20, 30	tail->next -> newNode	10 -> 20 -> 30 -> null
11	Update tail to newNode	10, 20, 30	tail -> newNode	10 -> 20 -> 30 -> null
12	End of enqueue operations	10, 20, 30	No changes	10 -> 20 -> 30 -> null

💡 All enqueue operations completed; tail points to last node; list updated accordingly.

Variable Tracker

Variable	Start	After Step 3	After Step 7	After Step 11	Final
head	NULL	Node(10)	Node(10)	Node(10)	Node(10)
tail	NULL	Node(10)	Node(20)	Node(30)	Node(30)
newNode	NULL	Node(10)	Node(20)	Node(30)	Node(30)
List Size	0	1	2	3	3

Key Moments - 3 Insights

Why do we check if tail is NULL before adding a new node?

Because if tail is NULL, the queue is empty, so both head and tail must point to the new node. This is shown in execution_table step 2 and 3.

Why do we update tail->next before moving tail to newNode?

We must link the current last node's next pointer to the new node before updating tail, so the list stays connected. See steps 6 and 7 in execution_table.

What happens if we forget to set newNode->next to NULL?

The new node might point to garbage or old data, breaking the list. The code sets newNode->next = NULL at step 1 and 4 and 8 to avoid this.

Visual Quiz - 3 Questions

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

A NULL

B Node with data 10

C Node with data 20

D Node with data 30

💡 Hint

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

At which step does the tail pointer first move from Node(10) to Node(20)?

A Step 3

B Step 6

C Step 7

D Step 10

💡 Hint

Look at the 'Pointer Changes' column for tail updates.

If we enqueue a new node with data=40 after step 11, what will tail point to?

A Node with data 40

B Node with data 30

C Node with data 20

D NULL

💡 Hint

Tail always points to the last node added, see steps 7 and 11.

Concept Snapshot

Enqueue Using Linked List:
- Create new node with data and next=NULL
- If queue empty (tail==NULL), set head and tail to new node
- Else, set tail->next to new node and update tail
- Tail always points to last node
- List grows by adding nodes at tail end

Full Transcript

Enqueue operation in a linked list queue involves creating a new node with the given data and next pointer set to NULL. If the queue is empty, both head and tail pointers are set to this new node. Otherwise, the current tail's next pointer is linked to the new node, and then tail is updated to point to the new node. This process ensures the queue grows at the tail end, maintaining FIFO order. The execution table shows each step with pointer updates and the visual linked list state. Key moments clarify why checking tail for NULL is important, why tail->next must be updated before tail, and why newNode->next must be NULL to avoid list corruption.