Enqueue Operation in DSA C++ - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add a new node at the end of the queue.

DSA C++

void enqueue(int value) {
    Node* newNode = new Node(value);
    if (rear == nullptr) {
        front = rear = newNode;
        return;
    }
    rear->[1] = newNode;
    rear = newNode;
}

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

Alink

Bnext

Cchild

Dprev

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'prev' instead of 'next' pointer.

Not updating the rear pointer after adding the new node.

The 'next' pointer of the current rear node should point to the new node to add it at the end.

2fill in blank

medium

Complete the code to check if the queue is empty before enqueueing.

DSA C++

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

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

Afront

Brear

CnewNode

Dtemp

Attempts:

3 left

💡 Hint

Common Mistakes

Checking 'newNode' instead of queue pointers.

Checking 'rear' which might not be updated yet.

Checking if 'front' is nullptr tells us if the queue is empty.

3fill in blank

hard

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

DSA C++

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

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

Arear

Bfront

CnewNode

Dtemp

Attempts:

3 left

💡 Hint

Common Mistakes

Updating front instead of rear.

Not updating any pointer after adding the node.

The rear pointer must be updated to the new node after adding it.

4fill in blank

hard

Fill both blanks to complete the enqueue function that handles empty and non-empty queues.

DSA C++

void enqueue(int value) {
    Node* newNode = new Node(value);
    if ([1] == nullptr) {
        [2] = newNode;
        rear = newNode;
        return;
    }
    rear->next = newNode;
    rear = newNode;
}

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

Afront

Brear

Dtemp

Attempts:

3 left

💡 Hint

Common Mistakes

Using rear in the condition instead of front.

Not setting front when queue is empty.

Check if front is nullptr to detect empty queue, then assign front to newNode.

5fill in blank

hard

Fill all three blanks to implement enqueue that updates pointers correctly and handles empty queue.

DSA C++

void enqueue(int value) {
    Node* newNode = new Node(value);
    if ([1] == nullptr) {
        [2] = newNode;
        rear = newNode;
        return;
    }
    rear->[3] = newNode;
    rear = newNode;
}

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

Afront

Cnext

Dprev

Attempts:

3 left

💡 Hint

Common Mistakes

Using rear instead of front in the condition.

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

Check if front is nullptr, set front to newNode, and link rear->next to newNode.

Practice

1. What does the enqueue operation do in a queue data structure?

easy

A. Returns the size of the queue

B. Removes an element from the front of the queue

C. Checks if the queue is empty

D. Adds an element to the end of the queue

Solution

1. Step 1: Understand the queue structure

   A queue works like a line where elements are added at the back and removed from the front.

2. Step 2: Define the enqueue operation

   Enqueue means adding a new element to the end of the queue to wait its turn.

3. Final Answer:

   Adds an element to the end of the queue -> Option D

4. Quick Check:

   Enqueue = Add at end [OK]

Hint: Enqueue always adds at the back of the queue [OK]

Common Mistakes:

• Confusing enqueue with dequeue (removal)
• Thinking enqueue removes elements
• Mixing enqueue with checking queue size

2. Which of the following is the correct syntax to enqueue an element 5 into a queue named q in C++ STL?

easy

A. q.push(5);

B. q.add(5);

C. q.enqueue(5);

D. q.insert(5);

Solution

1. Step 1: Recall C++ STL queue methods

   The standard queue in C++ uses push() to add elements at the back.

2. Step 2: Match the correct method

   enqueue is not a method in STL queue; push is used instead.

3. Final Answer:

   q.push(5); -> Option A

4. Quick Check:

   STL queue uses push() to enqueue [OK]

Hint: Use push() to add elements in C++ STL queue [OK]

Common Mistakes:

• Using enqueue() which is not in STL queue
• Using add() or insert() which are not queue methods
• Confusing push() with pop()

3. What will be the output of the following C++ code snippet?

std::queue<int> q;
q.push(10);
q.push(20);
q.push(30);
q.pop();
std::cout << q.front();

medium

A. 10

B. Compilation error

C. 20

D. 30

Solution

1. Step 1: Trace the enqueue operations

   Elements 10, 20, 30 are added in order: queue = [10, 20, 30]

2. Step 2: Apply the dequeue operation

   pop() removes the front element 10, so queue = [20, 30]

3. Step 3: Check the front element

   front() now returns 20, the new front.

4. Final Answer:

   20 -> Option C

5. Quick Check:

   After pop, front = 20 [OK]

Hint: pop removes front; front() shows new front [OK]

Common Mistakes:

• Assuming front remains 10 after pop
• Confusing pop with removing from back
• Expecting output 30 instead of 20

4. Identify the error in the following code that tries to enqueue an element into a queue:

std::queue<int> q;
q.enqueue(15);

medium

A. Missing #include <queue>

B. enqueue is not a member function of std::queue

C. Queue must be initialized with size

D. Cannot enqueue integer values

Solution

1. Step 1: Check std::queue methods

   std::queue does not have an enqueue method; it uses push to add elements.

2. Step 2: Identify the error cause

   Calling enqueue causes a compilation error because the method does not exist.

3. Final Answer:

   enqueue is not a member function of std::queue -> Option B

4. Quick Check:

   Use push() not enqueue() in std::queue [OK]

Hint: std::queue uses push(), not enqueue() [OK]

Common Mistakes:

• Assuming enqueue() exists in std::queue
• Thinking queue needs size at initialization
• Believing integers cannot be enqueued

5. You have a queue initially empty. You perform these operations:

enqueue(1)
enqueue(2)
dequeue()
enqueue(3)
dequeue()
enqueue(4)

What is the final state of the queue (front to back)?

hard

A. 3 -> 4 -> null

B. 2 -> 3 -> 4 -> null

C. 1 -> 3 -> 4 -> null

D. 4 -> null

Solution

1. Step 1: Perform operations step-by-step

   Start empty: []
   enqueue(1): [1]
   enqueue(2): [1, 2]
   dequeue(): removes 1 -> [2]
   enqueue(3): [2, 3]
   dequeue(): removes 2 -> [3]
   enqueue(4): [3, 4]

2. Step 2: Write final queue state

   Queue front is 3, then 4, then null (end).

3. Final Answer:

   3 -> 4 -> null -> Option A

4. Quick Check:

   After operations, queue = 3,4 [OK]

Hint: Track each enqueue/dequeue step carefully [OK]

Common Mistakes:

• Forgetting to remove elements on dequeue
• Mixing order of elements after operations
• Assuming dequeue removes from back