Queue Implementation Using Linked List in DSA C - Execution Trace

Concept Flow - Queue Implementation Using Linked List

Create new node with data

↓

Is queue empty?

Yes→Set front and rear to new node

|No↓

Link rear.next to new node

↓

Update rear to new node

↓

Done

This flow shows how a new element is added to the queue using a linked list by creating a node and updating pointers.

Execution Sample

DSA C

enqueue(10);
enqueue(20);
dequeue();
enqueue(30);

This code adds 10 and 20 to the queue, removes one element, then adds 30.

Execution Table

Step	Operation	Nodes in List	Pointer Changes	Visual State
1	enqueue(10)	10	front -> 10, rear -> 10	10 -> null
2	enqueue(20)	10 -> 20	rear.next -> 20, rear -> 20	10 -> 20 -> null
3	dequeue()	20	front -> 20	20 -> null
4	enqueue(30)	20 -> 30	rear.next -> 30, rear -> 30	20 -> 30 -> null
5	dequeue()	30	front -> 30	30 -> null
6	dequeue()	empty	front -> null, rear -> null	empty
7	dequeue()	empty	No change (queue empty)	empty

💡 Execution stops after dequeue on empty queue; no nodes left.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	After Step 7
front	null	Node(10)	Node(10)	Node(20)	Node(20)	Node(30)	null	null
rear	null	Node(10)	Node(20)	Node(20)	Node(30)	Node(30)	null	null
size	0	1	2	1	2	1	0	0

Key Moments - 3 Insights

Why do we update both front and rear when the queue is empty during enqueue?

When the queue is empty (see Step 1), both front and rear must point to the new node because it is the only element. This is shown in execution_table row 1.

What happens to the rear pointer when we dequeue the last element?

When the last element is dequeued (Step 6), both front and rear are set to null to indicate the queue is empty, as shown in execution_table row 6.

Why does dequeue on an empty queue not change pointers?

Dequeue on an empty queue (Step 7) does nothing because there are no nodes to remove. Pointers remain null, as shown in execution_table row 7.

Visual Quiz - 3 Questions

Look at the execution_table at Step 2, what is the visual state of the queue?

A 10 -> 20 -> null

B 20 -> 10 -> null

C 10 -> null

D 20 -> null

💡 Hint

Check the 'Visual State' column at Step 2 in the execution_table.

At which step does the queue become empty?

A Step 5

B Step 6

C Step 3

D Step 7

💡 Hint

Look at the 'Nodes in List' and 'Visual State' columns to find when the queue is empty.

If we enqueue(40) after Step 6, what will be the front pointer?

A null

B Node(30)

C Node(40)

D Node(20)

💡 Hint

After the queue is empty, enqueue sets front and rear to the new node.

Concept Snapshot

Queue using linked list:
- Enqueue: create node, add at rear, update rear pointer
- Dequeue: remove node from front, update front pointer
- If empty, front and rear are null
- Visual: front -> ... -> rear -> null
- Size tracks number of elements

Full Transcript

This visualization shows how a queue is implemented using a linked list. Enqueue adds a new node at the rear. If the queue is empty, both front and rear point to the new node. Dequeue removes the node at the front. When the last node is removed, both pointers become null. The execution table tracks each step, showing nodes, pointer changes, and the visual state of the queue. Variable tracker shows how front, rear, and size change over time. Key moments clarify common confusions about pointer updates and empty queue behavior.