Dequeue Using Linked List in DSA C++ - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a node structure for the dequeue.

DSA C++

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

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

Atail

Bhead

Cprev

Dlink

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'head' or 'tail' as a pointer inside the node instead of 'prev'.

Using a single pointer instead of two pointers for doubly linked list nodes.

The node in a doubly linked list has a pointer to the previous node called prev.

2fill in blank

medium

Complete the code to insert a new node at the front of the dequeue.

DSA C++

void insertFront(int value) {
    Node* newNode = new Node();
    newNode->data = value;
    newNode->prev = nullptr;
    newNode->next = head;
    if (head != nullptr) {
        head->[1] = newNode;
    }
    head = newNode;
    if (tail == nullptr) {
        tail = newNode;
    }
}

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

Aprev

Bnext

Chead

Dtail

Attempts:

3 left

💡 Hint

Common Mistakes

Updating the old head's next pointer instead of prev.

Not updating the old head's pointers at all.

When inserting at the front, the old head's previous pointer should point to the new node.

3fill in blank

hard

Fix the error in the code to delete a node from the rear of the dequeue.

DSA C++

void deleteRear() {
    if (tail == nullptr) return;
    Node* temp = tail;
    tail = tail->prev;
    if (tail != nullptr) {
        tail->[1] = nullptr;
    } else {
        head = nullptr;
    }
    delete temp;
}

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

Atail

Bnext

Chead

Dprev

Attempts:

3 left

💡 Hint

Common Mistakes

Setting tail->prev to nullptr instead of tail->next.

Not updating pointers after deletion.

After moving tail back, its next pointer should be set to nullptr to mark the end.

4fill in blank

hard

Fill both blanks to insert a new node at the rear of the dequeue.

DSA C++

void insertRear(int value) {
    Node* newNode = new Node();
    newNode->data = value;
    newNode->next = nullptr;
    newNode->[1] = tail;
    if (tail != nullptr) {
        tail->[2] = newNode;
    }
    tail = newNode;
    if (head == nullptr) {
        head = newNode;
    }
}

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

Aprev

Bnext

Chead

Dtail

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up next and prev pointers.

Not updating the old tail's next pointer.

The new node's previous pointer should point to the old tail, and the old tail's next pointer should point to the new node.

5fill in blank

hard

Fill all three blanks to delete a node from the front of the dequeue safely.

DSA C++

void deleteFront() {
    if (head == nullptr) return;
    Node* temp = head;
    head = head->[1];
    if (head != nullptr) {
        head->[2] = nullptr;
    } else {
        tail = [3];
    }
    delete temp;
}

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

Anext

Bprev

Cnullptr

Dhead

Attempts:

3 left

💡 Hint

Common Mistakes

Setting tail to head instead of nullptr when list is empty.

Not clearing the new head's previous pointer.

After moving head forward, set the new head's previous pointer to nullptr. If the list becomes empty, set tail to nullptr.

Practice

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

easy

A. It allows insertion and deletion from both ends in O(1) time without shifting elements.

B. It uses less memory than an array.

C. It automatically sorts the elements.

D. It restricts insertion to only one end.

Solution

1. Step 1: Understand Dequeue operations

   A dequeue allows insertion and deletion from both front and rear ends.

2. Step 2: Compare linked list and array

   Linked list supports O(1) insertion/deletion at both ends without shifting, unlike arrays.

3. Final Answer:

   It allows insertion and deletion from both ends in O(1) time without shifting elements. -> Option A

4. Quick Check:

   Linked list dequeue = O(1) insert/delete both ends [OK]

Hint: Linked list avoids shifting, so both ends are fast [OK]

Common Mistakes:

• Thinking arrays allow O(1) insert/delete at both ends
• Assuming linked list sorts elements automatically
• Confusing memory usage advantages

2. Which of the following is the correct syntax to insert an element at the front of a dequeue implemented using a linked list in C++?

easy

A. void insertFront(int data) { front = data; }

B. void insertFront(int data) { Node* newNode = new Node(data); newNode->next = front; front = newNode; }

C. void insertFront(int data) { front->data = data; }

D. void insertFront(int data) { delete front; front = new Node(data); }

Solution

1. Step 1: Create new node with data

   We allocate a new node and assign the data to it.

2. Step 2: Link new node to current front and update front pointer

   Set newNode->next to current front, then update front to newNode.

3. Final Answer:

   void insertFront(int data) { Node* newNode = new Node(data); newNode->next = front; front = newNode; } -> Option B

4. Quick Check:

   New node points to old front, front updated [OK]

Hint: New node next points to old front, then front updated [OK]

Common Mistakes:

• Assigning data directly to front pointer
• Deleting front before inserting
• Not linking new node to old front

3. Consider the following code snippet for a dequeue using linked list:

Dequeue dq;
dq.insertRear(10);
dq.insertFront(20);
dq.insertRear(30);
dq.deleteFront();
dq.insertFront(40);
dq.deleteRear();
dq.display();

What will be the output of dq.display()?

medium

A. 40 -> 10 -> null

B. 40 -> 20 -> 10 -> null

C. 20 -> 40 -> null

D. 10 -> 20 -> 30 -> null

Solution

1. Step 1: Trace insertions and deletions step-by-step

   insertRear(10): 10 insertFront(20): 20 -> 10 insertRear(30): 20 -> 10 -> 30 deleteFront(): removes 20, now 10 -> 30 insertFront(40): 40 -> 10 -> 30 deleteRear(): removes 30, now 40 -> 10

2. Step 2: Final list after all operations

   40 -> 10 -> null

3. Final Answer:

   40 -> 10 -> null -> Option A

4. Quick Check:

   Trace insert/delete carefully for final list [OK]

Hint: Trace each operation stepwise to find final list [OK]

Common Mistakes:

• Forgetting to update front or rear after deletion
• Mixing order of insertFront and insertRear
• Assuming deleted elements remain

4. The following code snippet for deleting from rear in a dequeue using linked list has a bug:

void deleteRear() {
  if (rear == nullptr) return;
  Node* temp = front;
  while (temp->next != rear) {
    temp = temp->next;
  }
  delete rear;
  rear = temp;
  rear->next = nullptr;
}

What is the bug in this code?

medium

A. It sets rear->next to non-null value.

B. It deletes the front node instead of rear.

C. It does not handle the case when there is only one node in the dequeue.

D. It causes memory leak by not deleting any node.

Solution

1. Step 1: Analyze the loop for single node case

   If only one node exists, front == rear, so while loop condition temp->next != rear fails immediately, temp = front.

2. Step 2: Check what happens after loop

   Deleting rear node, then setting rear = temp (which is front). But if only one node, temp == rear, so deleting rear deletes the only node, then rear points to deleted memory, causing dangling pointer.

3. Final Answer:

   It does not handle the case when there is only one node in the dequeue. -> Option C

4. Quick Check:

   Single node case needs special handling [OK]

Hint: Check single-node case separately before loop [OK]

Common Mistakes:

• Ignoring single-node edge case
• Assuming loop always runs at least once
• Not updating rear pointer correctly

5. You want to implement a function reverseDequeue() that reverses the order of elements in a dequeue implemented using a linked list. Which approach is best to reverse the dequeue efficiently?

hard

A. Delete all nodes and re-insert them in reverse order.

B. Create a new linked list and insert elements from the original dequeue at the rear one by one.

C. Use an auxiliary stack to push all elements, then pop and insert back into the dequeue.

D. Traverse the linked list once, reversing the next pointers of each node, then swap front and rear pointers.

Solution

1. Step 1: Reverse linked list pointers in one traversal

   Reversing next pointers of each node in one pass efficiently reverses the list.

2. Step 2: Swap front and rear pointers

   After reversal, front and rear pointers must be swapped to reflect new ends.

3. Final Answer:

   Traverse the linked list once, reversing the next pointers of each node, then swap front and rear pointers. -> Option D

4. Quick Check:

   In-place reversal with pointer swap is most efficient [OK]

Hint: Reverse pointers in one pass, then swap front and rear [OK]

Common Mistakes:

• Using extra memory unnecessarily
• Re-inserting nodes instead of reversing pointers
• Not swapping front and rear after reversal