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.

Practice

1. What is the main advantage of implementing a queue using a linked list instead of an array?

easy

A. Uses less memory than arrays always

B. Faster access to elements by index

C. Simpler code with fewer pointers

D. Dynamic size allows efficient memory use without fixed limits

Solution

1. Step 1: Understand queue size constraints in arrays

   Arrays have fixed size, so queues implemented with arrays have a maximum capacity.

2. Step 2: Analyze linked list flexibility

   Linked lists can grow or shrink dynamically, so queues using linked lists can handle any number of elements limited only by memory.

3. Final Answer:

   Dynamic size allows efficient memory use without fixed limits -> Option D

4. Quick Check:

   Linked list queues grow dynamically = A [OK]

Hint: Linked list queues grow dynamically, arrays don't [OK]

Common Mistakes:

• Thinking arrays always use less memory
• Confusing access speed with size flexibility
• Assuming linked lists have simpler code

2. Which of the following is the correct way to declare a node structure for a queue using a linked list in C?

easy

A. typedef struct Node { int data; Node* next; };

B. struct Node { int data; int next; };

C. typedef struct Node { int data; struct Node* next; } Node;

D. struct Node { int data; struct Node next; };

Solution

1. Step 1: Check pointer usage in struct

   The next pointer must be a pointer to struct Node, so 'struct Node* next;' is correct.

2. Step 2: Validate typedef syntax

   Using typedef with struct Node allows using 'Node' as type directly, which is correct in typedef struct Node { int data; struct Node* next; } Node;.

3. Final Answer:

   typedef struct Node { int data; struct Node* next; } Node; -> Option C

4. Quick Check:

   Pointer to struct Node with typedef = A [OK]

Hint: Use 'struct Node* next;' for pointer inside struct [OK]

Common Mistakes:

• Using 'int next;' instead of pointer
• Omitting 'struct' keyword before Node*
• Declaring next as struct Node instead of pointer

3. Consider the following C code snippet implementing enqueue operation for a queue using linked list:

void enqueue(Node** front, Node** rear, int value) {
    Node* newNode = (Node*)malloc(sizeof(Node));
    newNode->data = value;
    newNode->next = NULL;
    if (*rear == NULL) {
        *front = *rear = newNode;
    } else {
        (*rear)->next = newNode;
        *rear = newNode;
    }
}

What will be the state of the queue after these operations? enqueue(&front, &rear, 10); enqueue(&front, &rear, 20); enqueue(&front, &rear, 30);

medium

A. 10 -> 20 -> 30 -> NULL

B. 30 -> 20 -> 10 -> NULL

C. 10 -> 30 -> 20 -> NULL

D. 20 -> 10 -> 30 -> NULL

Solution

1. Step 1: Trace first enqueue call

   Queue is empty, so front and rear point to new node with data 10.

2. Step 2: Trace second and third enqueue calls

   New nodes with 20 and 30 are added at rear, linked in order: 10 -> 20 -> 30 -> NULL.

3. Final Answer:

   10 -> 20 -> 30 -> NULL -> Option A

4. Quick Check:

   Enqueue adds at rear, order preserved = C [OK]

Hint: Enqueue adds at rear, so order is insertion order [OK]

Common Mistakes:

• Confusing enqueue with push (stack behavior)
• Reversing order of nodes
• Not updating rear pointer correctly

4. The following dequeue function for a queue using linked list has a bug. Identify the bug:

int dequeue(Node** front, Node** rear) {
    if (*front == NULL) {
        printf("Queue is empty\n");
        return -1;
    }
    Node* temp = *front;
    int value = temp->data;
    *front = (*front)->next;
    free(temp);
    return value;
}

medium

A. Does not free memory of dequeued node

B. Does not update rear when queue becomes empty

C. Returns wrong value after dequeue

D. Does not check if front is NULL before dequeue

Solution

1. Step 1: Analyze dequeue steps

   Function removes front node and updates front pointer correctly.

2. Step 2: Check rear pointer update

   If after dequeue front becomes NULL (queue empty), rear should also be set to NULL but this is missing.

3. Final Answer:

   Does not update rear when queue becomes empty -> Option B

4. Quick Check:

   Rear must be NULL if queue empty = B [OK]

Hint: Set rear to NULL when front becomes NULL after dequeue [OK]

Common Mistakes:

• Forgetting to update rear on empty queue
• Not freeing dequeued node memory
• Returning wrong data value

5. You want to implement a function reverseQueue that reverses the elements of a queue implemented using a linked list. Which approach is the most efficient and correct?

hard

A. Use a stack to push all elements, then enqueue them back to queue

B. Traverse the linked list and swap data of front and rear nodes repeatedly

C. Dequeue all elements and enqueue them back in the same order

D. Create a new linked list and copy elements in reverse order without modifying original

Solution

1. Step 1: Understand reversing queue with linked list

   Reversing requires changing order of elements, which is not straightforward by swapping nodes in singly linked list.

2. Step 2: Evaluate stack approach

   Using a stack to push all elements then enqueue back reverses order efficiently and correctly.

3. Step 3: Check other options

   Swapping data repeatedly is complex and error-prone; copying to new list wastes memory; re-enqueueing same order does nothing.

4. Final Answer:

   Use a stack to push all elements, then enqueue them back to queue -> Option A

5. Quick Check:

   Stack reverses order efficiently = D [OK]

Hint: Use stack to reverse queue elements easily [OK]

Common Mistakes:

• Trying to swap nodes in singly linked list
• Re-enqueueing without changing order
• Copying to new list wastes memory