Queue Implementation Using Linked List in DSA C - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a new node with given data.

DSA C

struct Node* newNode(int data) {
    struct Node* temp = (struct Node*)malloc(sizeof(struct Node));
    temp->data = [1];
    temp->next = NULL;
    return temp;
}

Drag options to blanks, or click blank then click option'

Adata

BNULL

Ctemp

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning NULL or 0 instead of the data value.

Using the wrong variable name.

The new node's data field should be set to the input parameter 'data'.

2fill in blank

medium

Complete the code to check if the queue is empty.

DSA C

int isEmpty(struct Queue* q) {
    return (q->front == [1]);
}

Drag options to blanks, or click blank then click option'

Aq->rear

B0

CNULL

Dq->front

Attempts:

3 left

💡 Hint

Common Mistakes

Comparing front to rear instead of NULL.

Using 0 instead of NULL in pointer comparison.

The queue is empty if the front pointer is NULL.

3fill in blank

hard

Fix the error in the enqueue function to correctly update the rear pointer.

DSA C

void enqueue(struct Queue* q, int data) {
    struct Node* temp = newNode(data);
    if (q->rear == NULL) {
        q->front = temp;
        q->rear = [1];
        return;
    }
    q->rear->next = temp;
    q->rear = temp;
}

Drag options to blanks, or click blank then click option'

Atemp

Bq->front

CNULL

Dq->rear

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning q->front or NULL to q->rear.

Not updating q->rear at all.

When the queue is empty, both front and rear should point to the new node 'temp'.

4fill in blank

hard

Fill both blanks to correctly dequeue an element and update the queue pointers.

DSA C

int dequeue(struct Queue* q) {
    if (isEmpty(q))
        return -1;
    struct Node* temp = q->front;
    int data = temp->data;
    q->front = [1];
    if (q->front == NULL)
        q->rear = [2];
    free(temp);
    return data;
}

Drag options to blanks, or click blank then click option'

Aq->front->next

BNULL

Ctemp->next

Dq->rear->next

Attempts:

3 left

💡 Hint

Common Mistakes

Using q->front->next before updating front pointer.

Not setting rear to NULL when queue becomes empty.

After removing the front node, front should point to the next node (temp->next). If front becomes NULL, rear should also be set to NULL.

5fill in blank

hard

Fill all three blanks to print all elements in the queue from front to rear.

DSA C

void displayQueue(struct Queue* q) {
    struct Node* temp = [1];
    while (temp != [2]) {
        printf("%d -> ", [3]);
        temp = temp->next;
    }
    printf("NULL\n");
}

Drag options to blanks, or click blank then click option'

Aq->front

BNULL

Ctemp->data

Dq->rear

Attempts:

3 left

💡 Hint

Common Mistakes

Starting from rear instead of front.

Looping until temp equals rear instead of NULL.

Printing temp instead of temp->data.

Start from the front node, loop until temp is NULL, and print temp->data at each step.

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