Delete from Beginning of Doubly Linked List in DSA C++ - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to delete the first node from the doubly linked list.

DSA C++

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

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

Adata

Bnext

Cprev

Dhead

Attempts:

3 left

💡 Hint

Common Mistakes

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

Not updating head->prev to nullptr after deletion.

To delete the first node, move the head pointer to the next node using head->next.

2fill in blank

medium

Complete the code to update the previous pointer of the new head after deletion.

DSA C++

void deleteBeginning(Node*& head) {
    if (head == nullptr) return;
    Node* temp = head;
    head = head->next;
    if (head != nullptr) {
        head->[1] = nullptr;
    }
    delete temp;
}

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

Aprev

Bnext

Cdata

Dhead

Attempts:

3 left

💡 Hint

Common Mistakes

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

Not checking if head is nullptr before updating.

After moving head to the next node, set head->prev to nullptr to mark it as the first node.

3fill in blank

hard

Fix the error in the code that deletes the first node of the doubly linked list.

DSA C++

void deleteBeginning(Node*& head) {
    if (head == nullptr) return;
    Node* temp = head;
    head = head->next;
    if (head != nullptr) {
        head->prev = [1];
    }
    delete temp;
}

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

Ahead

Btemp

Cnullptr

Dhead->prev

Attempts:

3 left

💡 Hint

Common Mistakes

Setting head->prev to head or temp instead of nullptr.

Forgetting to update head->prev after deletion.

The previous pointer of the new head must be set to nullptr, not to head or any other pointer.

4fill in blank

hard

Fill both blanks to correctly delete the first node and update pointers.

DSA C++

void deleteBeginning(Node*& head) {
    if (head == nullptr) return;
    Node* temp = head;
    head = head->[1];
    if (head != nullptr) {
        head->[2] = nullptr;
    }
    delete temp;
}

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

Anext

Bprev

Cdata

Dhead

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping next and prev pointers.

Not setting head->prev to nullptr.

Move head to head->next and set head->prev to nullptr to maintain list integrity.

5fill in blank

hard

Fill all three blanks to implement deletion of the first node in a doubly linked list.

DSA C++

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

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

Atemp

Bnext

Cprev

Dhead

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong variable names.

Not updating pointers correctly.

Use a temporary pointer 'temp' to hold the old head, move head to head->next, and set head->prev to nullptr.

Practice

1. What happens to the head pointer when you delete the first node from a doubly linked list?

easy

A. It points to the last node

B. It becomes null immediately

C. It moves to the second node in the list

D. It remains pointing to the deleted node

Solution

1. Step 1: Understand the head pointer role

   The head points to the first node in the doubly linked list.

2. Step 2: After deletion, update the head pointer

   When the first node is deleted, the head must point to the next node, which becomes the new first node.

3. Final Answer:

   It moves to the second node in the list -> Option C

4. Quick Check:

   Head points to new first node after deletion [OK]

Hint: Head always moves to next node after deleting first [OK]

Common Mistakes:

• Not updating head pointer after deletion
• Setting head to null without checking list size
• Leaving head pointing to deleted node

2. Which of the following is the correct syntax to delete the first node in a doubly linked list in C++?

easy

A. head = head->next; delete head->prev; head->prev = nullptr;

B. Node* temp = head; head = head->next; head->prev = nullptr; delete temp;

C. head->prev = nullptr; delete head; head = head->next;

D. delete head; head = head->next; head->prev = nullptr;

Solution

1. Step 1: Store current head in temp

   We save the current first node in a temporary pointer to delete it safely after updating head.

2. Step 2: Update head and fix previous pointer

   Set head to the next node, then set head->prev to nullptr to detach the old first node.

3. Step 3: Delete the old first node

   Finally, delete the node stored in temp to free memory.

4. Final Answer:

   Node* temp = head; head = head->next; head->prev = nullptr; delete temp; -> Option B

5. Quick Check:

   Proper order: save, update, fix pointers, delete [OK]

Hint: Save head first, update pointers, then delete saved node [OK]

Common Mistakes:

• Deleting head before updating pointers
• Not setting head->prev to nullptr
• Accessing head->prev when head is nullptr

3. What will be the output of the following code snippet after deleting the first node?

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

Node* head = new Node{1, nullptr, nullptr};
Node* second = new Node{2, head, nullptr};
head->next = second;

// Delete from beginning
Node* temp = head;
head = head->next;
head->prev = nullptr;
delete temp;

// Print list
Node* curr = head;
while(curr != nullptr) {
  std::cout << curr->data << " ";
  curr = curr->next;
}

medium

A. 1 2

B. No output (runtime error)

C. 1

D. 2

Solution

1. Step 1: Initial list setup

   List has two nodes: 1 (head) -> 2. Head points to node with data 1.

2. Step 2: Delete first node and update pointers

   Head moves to second node (data 2), and its prev is set to nullptr. The first node is deleted.

3. Step 3: Print remaining list

   Only node with data 2 remains, so output is "2".

4. Final Answer:

   2 -> Option D

5. Quick Check:

   After deletion, list starts at 2 [OK]

Hint: After deleting head, print starts from new head [OK]

Common Mistakes:

• Printing deleted node's data
• Not updating head->prev to nullptr
• Assuming both nodes print after deletion

4. Identify the error in the following code that deletes the first node of a doubly linked list:

void deleteBeginning(Node*& head) {
  if (head == nullptr) return;
  delete head;
  head = head->next;
  if (head != nullptr) head->prev = nullptr;
}

medium

A. Deleting head before updating head pointer causes undefined behavior

B. Missing check if head->next is nullptr

C. head->prev should be set to head->next

D. No error, code works fine

Solution

1. Step 1: Analyze deletion order

   The code deletes head first, then tries to access head->next, which is invalid after deletion.

2. Step 2: Correct order to avoid undefined behavior

   First save head->next in a temp pointer, update head, then delete old node.

3. Final Answer:

   Deleting head before updating head pointer causes undefined behavior -> Option A

4. Quick Check:

   Accessing deleted memory causes error [OK]

Hint: Always update pointers before deleting nodes [OK]

Common Mistakes:

• Deleting node before updating pointers
• Accessing deleted node's members
• Not checking for empty list

5. You have a doubly linked list with nodes: 10 <-> 20 <-> 30 <-> 40. After deleting the first node twice from the beginning, what will be the printed list?

hard

A. 30 <-> 40

B. 20 <-> 30 <-> 40

C. 10 <-> 20 <-> 30 <-> 40

D. 40

Solution

1. Step 1: Initial list nodes

   List is 10 <-> 20 <-> 30 <-> 40, head points to 10.

2. Step 2: First deletion

   Delete node 10, head moves to 20, set head->prev = nullptr. List now: 20 <-> 30 <-> 40.

3. Step 3: Second deletion

   Delete node 20, head moves to 30, set head->prev = nullptr. List now: 30 <-> 40.

4. Final Answer:

   30 <-> 40 -> Option A

5. Quick Check:

   Two deletions remove first two nodes [OK]

Hint: Each deletion moves head to next node [OK]

Common Mistakes:

• Deleting only once instead of twice
• Not updating head->prev after second deletion
• Printing old list without deletions