Queue Implementation Using Linked List in DSA C++ - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare the node structure for the linked list queue.

DSA C++

struct Node {
    int data;
    Node* [1];
};

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

Aprev

Bnext

Chead

Dtail

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'prev' instead of 'next' for the pointer.

Using 'head' or 'tail' as pointer names inside the node.

The 'next' pointer links to the next node in the queue.

2fill in blank

medium

Complete the code to initialize the queue's front and rear pointers to nullptr in the constructor.

DSA C++

class Queue {
private:
    Node* front;
    Node* rear;
public:
    Queue() {
        front = [1];
        rear = [1];
    }
};

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

Afront

BNULL

C0

Dnullptr

Attempts:

3 left

💡 Hint

Common Mistakes

Using NULL or 0 instead of nullptr.

Initializing rear to front instead of nullptr.

Both front and rear pointers should be initialized to nullptr to indicate an empty queue.

3fill in blank

hard

Fix the error in the enqueue function to correctly add a new node at the rear of the queue.

DSA C++

void enqueue(int value) {
    Node* temp = new Node();
    temp->data = value;
    temp->next = nullptr;
    if (rear == nullptr) {
        front = temp;
        rear = temp;
    } else {
        rear->[1] = temp;
        rear = temp;
    }
}

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

Afront

Bprev

Cnext

Ddata

Attempts:

3 left

💡 Hint

Common Mistakes

Using rear->prev instead of rear->next.

Assigning rear->data instead of rear->next.

The rear node's 'next' pointer should point to the new node to link it at the end.

4fill in blank

hard

Fill both blanks to correctly implement the dequeue function that removes the front node.

DSA C++

int dequeue() {
    if (front == nullptr) {
        return -1; // Queue is empty
    }
    Node* temp = front;
    int value = temp->data;
    front = front->[1];
    if (front == nullptr) {
        rear = [2];
    }
    delete temp;
    return value;
}

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

Anext

Bnullptr

Cfront

Dtemp

Attempts:

3 left

💡 Hint

Common Mistakes

Setting rear to front instead of nullptr when queue is empty.

Not updating front to front->next.

After removing the front node, front moves to front->next. If front becomes nullptr, rear must also be nullptr.

5fill in blank

hard

Fill all three blanks to implement a function that checks if the queue is empty.

DSA C++

bool isEmpty() {
    return [1] == [2];
}

// Usage example:
// if (queue.[3]()) { /* queue is empty */ }

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

Afront

Bnullptr

CisEmpty

Drear

Attempts:

3 left

💡 Hint

Common Mistakes

Comparing rear to nullptr instead of front.

Using wrong function name like empty instead of isEmpty.

The queue is empty if front equals nullptr. The function name is isEmpty.

Practice

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

easy

A. Dynamic size without fixed capacity

B. Faster access to middle elements

C. Uses less memory always

D. Simpler to implement than arrays

Solution

1. Step 1: Understand queue size limitation in arrays

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

2. Step 2: Linked list dynamic size

   Linked lists can grow or shrink dynamically, so queues using linked lists do not have fixed capacity limits.

3. Final Answer:

   Dynamic size without fixed capacity -> Option A

4. Quick Check:

   Linked list queues grow dynamically = D [OK]

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

Common Mistakes:

• Thinking linked lists use less memory always
• Confusing access speed advantages
• Assuming linked lists are simpler to implement

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

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

C. struct Node { int data; Node* prev; };

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

Solution

1. Step 1: Understand node structure for linked list

   A node must contain data and a pointer to the next node.

2. Step 2: Analyze options

   struct Node { int data; Node* next; }; correctly uses a pointer to next node. struct Node { int data; Node next; }; uses next as object, which is invalid. struct Node { int data; Node* prev; }; uses prev pointer, which is for doubly linked list. struct Node { int data; int next; }; uses int for next, which is incorrect.

3. Final Answer:

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

4. Quick Check:

   Node with data and next pointer = A [OK]

Hint: Node must have pointer to next node, not object or int [OK]

Common Mistakes:

• Using object instead of pointer for next
• Confusing singly and doubly linked list nodes
• Using int instead of pointer for next

3. What will be the output after performing these operations on an empty queue implemented using linked list?

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

Print the queue elements from front to rear.

medium

A. 20 -> 30 -> null

B. 10 -> 20 -> 30 -> null

C. 30 -> null

D. 10 -> 30 -> null

Solution

1. Step 1: Perform enqueue(10) and enqueue(20)

   Queue becomes: 10 -> 20 -> null

2. Step 2: Perform dequeue()

   Removes 10, queue becomes: 20 -> null

3. Step 3: Perform enqueue(30)

   Queue becomes: 20 -> 30 -> null

4. Step 4: Perform dequeue()

   Removes 20, queue becomes: 30 -> null

5. Final Answer:

   30 -> null -> Option C

6. Quick Check:

   After operations queue front is 30 [OK]

Hint: Track enqueue/dequeue stepwise to find final queue [OK]

Common Mistakes:

• Forgetting to remove front on dequeue
• Appending dequeue element instead of removing
• Mixing order of operations

4. Identify the error in this dequeue function for a queue implemented using linked list:

int dequeue() {
  if (front == nullptr) return -1;
  Node* temp = front;
  front = front->next;
  int val = temp->data;
  delete temp;
  return val;
}

medium

A. No error, function is correct

B. Does not update rear when queue becomes empty

C. Deletes front node before accessing data

D. Returns wrong value after deletion

Solution

1. Step 1: Analyze dequeue steps

   Function removes front node and returns its data correctly.

2. Step 2: Check rear pointer update

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

3. Final Answer:

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

4. Quick Check:

   Rear must be nullptr if queue empty [OK]

Hint: Update rear to nullptr when queue becomes empty [OK]

Common Mistakes:

• Ignoring rear pointer update on empty queue
• Deleting node before reading data
• Returning wrong value after deletion

5. You want to implement a function reverseQueue() that reverses the elements of a queue implemented using a linked list without using extra arrays or STL containers. Which approach is best?

hard

A. Use recursion to dequeue elements and enqueue them back in reverse order

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

C. Create a new linked list and insert elements at front while dequeuing

D. Use two pointers to reverse links between nodes iteratively

Solution

1. Step 1: Understand reversing linked list nodes

   Reversing links between nodes iteratively changes the queue order without extra space.

2. Step 2: Analyze options

   Use recursion to dequeue elements and enqueue them back in reverse order uses recursion but may use call stack (extra space). Traverse linked list and swap data of front and rear nodes repeatedly swaps data but is inefficient and complex. Create a new linked list and insert elements at front while dequeuing creates new list (extra space). Use two pointers to reverse links between nodes iteratively reverses links in place efficiently.

3. Final Answer:

   Use two pointers to reverse links between nodes iteratively -> Option D

4. Quick Check:

   In-place link reversal is best for linked list queue reverse [OK]

Hint: Reverse links in place with two pointers for best solution [OK]

Common Mistakes:

• Using extra arrays or containers
• Swapping data instead of links
• Using recursion causing extra space