Insert at Beginning of Doubly Linked List in DSA C++ - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a doubly linked list?

A doubly linked list is a chain of nodes where each node has three parts: data, a pointer to the next node, and a pointer to the previous node. This allows traversal in both directions.

beginner

What steps are needed to insert a new node at the beginning of a doubly linked list?

1. Create a new node with the given data.
2. Set the new node's next pointer to the current head.
3. Set the new node's previous pointer to null.
4. If the list is not empty, set the current head's previous pointer to the new node.
5. Update the head pointer to the new node.

intermediate

Why do we need to update the previous pointer of the old head node when inserting at the beginning?

Because the new node becomes the first node, the old head now comes after it. Its previous pointer must point back to the new node to maintain the doubly linked structure.

beginner

What happens if the doubly linked list is empty when inserting at the beginning?

The new node becomes the only node in the list. Its next and previous pointers are set to null, and the head points to this new node.

beginner

Show the C++ code snippet to insert a node at the beginning of a doubly linked list.

```cpp struct Node { int data; Node* next; Node* prev; }; void insertAtBeginning(Node*& head, int newData) { Node* newNode = new Node(); newNode->data = newData; newNode->prev = nullptr; newNode->next = head; if (head != nullptr) { head->prev = newNode; } head = newNode; } ```

What pointer of the new node should be set to nullptr when inserting at the beginning?

A none

B next

C both prev and next

D prev

The new node is the first node, so its previous pointer should be nullptr. Its next pointer points to the old head.

When inserting at the beginning, what must be updated in the old head node?

A Its prev pointer to new node

B Its data value

C Its next pointer to new node

D Nothing

The old head's previous pointer must point to the new node to maintain the doubly linked list structure.

If the list is empty, what will be the next pointer of the new node after insertion at beginning?

A nullptr

B Points to itself

C Points to old head

D Undefined

Since the list is empty, the new node's next pointer is nullptr.

Which of these is NOT a step in inserting at the beginning of a doubly linked list?

A Create new node

B Set new node's next to head

C Set head's next to new node

D Update head to new node

We do not set head's next to new node; instead, we set new node's next to head.

What is the time complexity of inserting a node at the beginning of a doubly linked list?

A O(n)

B O(1)

C O(log n)

D O(n^2)

Insertion at the beginning is done in constant time O(1) because it involves only pointer updates.

Explain the process of inserting a new node at the beginning of a doubly linked list.

Think about how pointers change when adding a new first node.

Why is it important to update the previous pointer of the old head node when inserting at the beginning?

Consider how doubly linked lists allow moving both ways.

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