Queue Implementation Using Linked List in DSA C++ - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Enqueue and Dequeue Operations

What is the printed state of the queue after the following operations?

Enqueue 10, Enqueue 20, Dequeue, Enqueue 30

DSA C++

struct Node {
    int data;
    Node* next;
    Node(int val) : data(val), next(nullptr) {}
};

class Queue {
    Node* front;
    Node* rear;
public:
    Queue() : front(nullptr), rear(nullptr) {}
    void enqueue(int val) {
        Node* newNode = new Node(val);
        if (!rear) {
            front = rear = newNode;
            return;
        }
        rear->next = newNode;
        rear = newNode;
    }
    void dequeue() {
        if (!front) return;
        Node* temp = front;
        front = front->next;
        if (!front) rear = nullptr;
        delete temp;
    }
    void print() {
        Node* temp = front;
        while (temp) {
            std::cout << temp->data << " -> ";
            temp = temp->next;
        }
        std::cout << "null" << std::endl;
    }
};

int main() {
    Queue q;
    q.enqueue(10);
    q.enqueue(20);
    q.dequeue();
    q.enqueue(30);
    q.print();
    return 0;
}

A 10 -> 30 -> null

B 20 -> 30 -> null

C 10 -> 20 -> 30 -> null

D 30 -> null

💡 Hint

Remember that dequeue removes the front element.

After enqueueing 10 and 20, the queue is 10 -> 20 -> null. Dequeue removes 10, so queue becomes 20 -> null. Enqueue 30 adds it to the rear, resulting in 20 -> 30 -> null.

🧠 Conceptual

intermediate

Queue Empty State Check

Which condition correctly checks if the queue implemented using linked list is empty?

A if (front == nullptr) { /* queue is empty */ }

B if (rear == nullptr) { /* queue is empty */ }

C if (front == rear) { /* queue is empty */ }

D if (front->next == nullptr) { /* queue is empty */ }

💡 Hint

Empty queue means no front node.

In a linked list queue, when front is nullptr, it means no elements are present. Rear can be nullptr or not, but front is the main indicator.

🔧 Debug

advanced

Identify the Bug in Dequeue Method

What error will occur when running this dequeue method if the queue has only one element?

DSA C++

void dequeue() {
    if (!front) return;
    Node* temp = front;
    front = front->next;
    delete temp;
}

A Dangling rear pointer causing undefined behavior

B No error, works fine

C Segmentation fault due to deleting null pointer

D Memory leak because node is not deleted

💡 Hint

Consider what happens to rear when the last element is dequeued.

When the last element is dequeued, front becomes nullptr but rear still points to the deleted node, causing dangling pointer issues.

❓ Predict Output

advanced

Output After Multiple Enqueue and Dequeue

What is the queue state after these operations?

Enqueue 5, Enqueue 15, Enqueue 25, Dequeue, Dequeue, Enqueue 35

DSA C++

struct Node {
    int data;
    Node* next;
    Node(int val) : data(val), next(nullptr) {}
};

class Queue {
    Node* front;
    Node* rear;
public:
    Queue() : front(nullptr), rear(nullptr) {}
    void enqueue(int val) {
        Node* newNode = new Node(val);
        if (!rear) {
            front = rear = newNode;
            return;
        }
        rear->next = newNode;
        rear = newNode;
    }
    void dequeue() {
        if (!front) return;
        Node* temp = front;
        front = front->next;
        if (!front) rear = nullptr;
        delete temp;
    }
    void print() {
        Node* temp = front;
        while (temp) {
            std::cout << temp->data << " -> ";
            temp = temp->next;
        }
        std::cout << "null" << std::endl;
    }
};

int main() {
    Queue q;
    q.enqueue(5);
    q.enqueue(15);
    q.enqueue(25);
    q.dequeue();
    q.dequeue();
    q.enqueue(35);
    q.print();
    return 0;
}

A 15 -> 25 -> 35 -> null

B 35 -> null

C 25 -> 35 -> null

D 25 -> null

💡 Hint

Dequeue removes from front, track carefully.

After enqueueing 5,15,25 queue is 5->15->25->null. Dequeue removes 5, then 15, queue is 25->null. Enqueue 35 adds to rear, queue is 25->35->null. But careful: after second dequeue, front is 25. Then enqueue 35 adds after 25. So queue is 25->35->null. The correct answer is option C.

🧠 Conceptual

expert

Time Complexity of Queue Operations Using Linked List

What is the time complexity of enqueue and dequeue operations in a queue implemented using a linked list with front and rear pointers?

A Both enqueue and dequeue are O(n)

B Enqueue is O(n), dequeue is O(1)

C Enqueue is O(1), dequeue is O(n)

D Both enqueue and dequeue are O(1)

💡 Hint

Think about how front and rear pointers help.

With front and rear pointers, enqueue adds at rear in O(1), dequeue removes from front in O(1). No traversal needed.

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