Insert at Beginning of Doubly Linked List in DSA C++ - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a new node with given data.

DSA C++

Node* newNode = new Node([1]);

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

Atemp

Bdata

Cnullptr

Dhead

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'head' instead of data when creating the new node.

Using nullptr which is for pointers, not data.

We create a new node with the given data value to insert at the beginning.

2fill in blank

medium

Complete the code to link the new node's next pointer to the current head.

DSA C++

newNode->next = [1];

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Ahead

BnewNode

Cnullptr

Dtemp

Attempts:

3 left

💡 Hint

Common Mistakes

Setting newNode->next to itself causing a cycle.

Setting newNode->next to nullptr which breaks the list.

The new node's next pointer should point to the current head node.

3fill in blank

hard

Fix the error in updating the previous pointer of the old head node.

DSA C++

if (head != nullptr) head->[1] = newNode;

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

Anext

Bdata

Chead

Dprev

Attempts:

3 left

💡 Hint

Common Mistakes

Updating next pointer instead of prev.

Trying to assign to data or head which are not pointers.

The previous pointer of the old head should point back to the new node.

4fill in blank

hard

Fill both blanks to update the head pointer and set the new node's previous pointer.

DSA C++

head = [1];
head->[2] = nullptr;

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

AnewNode

Bhead

Cprev

Dnext

Attempts:

3 left

💡 Hint

Common Mistakes

Setting head to old head instead of newNode.

Setting head->next to nullptr instead of prev.

The head pointer should point to the new node, and its previous pointer should be nullptr.

5fill in blank

hard

Fill all three blanks to complete the insert at beginning function.

DSA C++

void insertAtBeginning(Node*& head, int data) {
    Node* newNode = new Node([1]);
    newNode->next = [2];
    if (head != nullptr) head->prev = newNode;
    head = newNode;
    head->prev = [3];
}

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

Adata

Bhead

Cnullptr

DnewNode

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to update head pointer.

Setting head->prev to newNode instead of nullptr.

Create new node with data, link newNode->next to head, and set head->prev to nullptr.

Practice

1. What is the main purpose of inserting a node at the beginning of a doubly linked list?

easy

A. To delete the first node of the list

B. To add a new node before the current head node

C. To add a new node after the tail node

D. To reverse the entire list

Solution

1. Step 1: Understand the head node position

   The head node is the first node in the doubly linked list.

2. Step 2: Insert new node before head

   Inserting at the beginning means placing the new node before the current head and updating pointers.

3. Final Answer:

   To add a new node before the current head node -> Option B

4. Quick Check:

   Insert at beginning = Add before head [OK]

Hint: Insert at beginning means new node becomes the head [OK]

Common Mistakes:

• Confusing insertion at beginning with insertion at end
• Not updating the previous head's previous pointer
• Trying to delete instead of insert

2. Which of the following is the correct syntax to create a new node and insert it at the beginning of a doubly linked list in C++?

easy

A. Node* newNode = new Node(data); head->prev = newNode; newNode->next = head; head = newNode;

B. Node newNode = Node(data); newNode->next = head; head = newNode;

C. Node* newNode = new Node(data); head->next = newNode; newNode->prev = head; head = newNode;

D. Node* newNode = new Node(data); newNode->next = head; head = newNode;

Solution

1. Step 1: Create new node with pointer

   Use pointer syntax: Node* newNode = new Node(data);

2. Step 2: Update pointers for doubly linked list

   Set newNode->next to current head, update head->prev to newNode, then update head to newNode.

3. Final Answer:

   Node* newNode = new Node(data); head->prev = newNode; newNode->next = head; head = newNode; -> Option A

4. Quick Check:

   Correct pointer updates = Node* newNode = new Node(data); head->prev = newNode; newNode->next = head; head = newNode; [OK]

Hint: Remember to update both next and prev pointers [OK]

Common Mistakes:

• Using object instead of pointer for new node
• Not updating head->prev pointer
• Swapping next and prev pointers incorrectly

3. What will be the output of the following code snippet after inserting 10 at the beginning of the doubly linked list?

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

Node* head = new Node(20);
head->next = new Node(30);
head->next->prev = head;

Node* newNode = new Node(10);
newNode->next = head;
head->prev = newNode;
head = newNode;

// Print list from head
Node* temp = head;
while(temp) {
  std::cout << temp->data << " -> ";
  temp = temp->next;
}
std::cout << "null";

medium

A. 30 -> 20 -> 10 -> null

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

C. 10 -> 20 -> 30 -> null

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

Solution

1. Step 1: Initial list setup

   List initially: 20 <-> 30, head points to 20.

2. Step 2: Insert 10 at beginning

   New node 10 points next to 20, 20's prev points to 10, head updated to 10.

3. Step 3: Print list from head

   Traverse from head: 10 -> 20 -> 30 -> null

4. Final Answer:

   10 -> 20 -> 30 -> null -> Option C

5. Quick Check:

   New head is 10, followed by 20 and 30 [OK]

Hint: New node becomes head, old head shifts right [OK]

Common Mistakes:

• Not updating head pointer after insertion
• Incorrect pointer assignments causing wrong order
• Printing from tail instead of head

4. Identify the error in the following code snippet that inserts a node at the beginning of a doubly linked list:

void insertAtBeginning(Node*& head, int data) {
  Node* newNode = new Node(data);
  newNode->next = head;
  head->prev = newNode;
  head = newNode;
}

medium

A. newNode->prev should be set to head

B. newNode->next should be set to nullptr

C. head pointer should not be passed by reference

D. Missing check if head is nullptr before accessing head->prev

Solution

1. Step 1: Analyze pointer usage

   Code accesses head->prev without checking if head is nullptr (empty list).

2. Step 2: Identify potential runtime error

   If head is nullptr, accessing head->prev causes a crash (null pointer dereference).

3. Final Answer:

   Missing check if head is nullptr before accessing head->prev -> Option D

4. Quick Check:

   Always check head != nullptr before head->prev [OK]

Hint: Check if list is empty before updating pointers [OK]

Common Mistakes:

• Ignoring nullptr checks causing crashes
• Incorrect pointer assignments for prev
• Passing head by value instead of reference

5. Given a doubly linked list with nodes containing values [5, 15, 25], write the sequence of node values after inserting 1 at the beginning and then 35 at the beginning. What is the final list?

hard

A. 35 -> 1 -> 5 -> 15 -> 25 -> null

B. 1 -> 35 -> 5 -> 15 -> 25 -> null

C. 5 -> 15 -> 25 -> 1 -> 35 -> null

D. 35 -> 5 -> 15 -> 25 -> 1 -> null

Solution

1. Step 1: Initial list

   List is 5 -> 15 -> 25 -> null.

2. Step 2: Insert 1 at beginning

   New head is 1, list becomes 1 -> 5 -> 15 -> 25 -> null.

3. Step 3: Insert 35 at beginning

   New head is 35, list becomes 35 -> 1 -> 5 -> 15 -> 25 -> null.

4. Final Answer:

   35 -> 1 -> 5 -> 15 -> 25 -> null -> Option A

5. Quick Check:

   Last inserted node is new head [OK]

Hint: Last inserted node at beginning becomes new head [OK]

Common Mistakes:

• Inserting nodes at end instead of beginning
• Not updating head pointer after each insertion
• Mixing order of inserted nodes