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 moving forward and backward through the list.

beginner

What happens when you insert a node at the beginning of a doubly linked list?

The new node becomes the head (first node). Its next pointer points to the old head, and the old head's previous pointer points back to the new node.

intermediate

In C#, what pointers do you update when inserting at the beginning of a doubly linked list?

You update the new node's next pointer to the current head, the current head's previous pointer to the new node, and then set the head to the new node.

beginner

Why do we check if the list is empty before inserting at the beginning?

Because if the list is empty, the new node becomes the head and tail, and its next and previous pointers are null. No other nodes need updating.

intermediate

Show the steps to insert a node with value 5 at the beginning of a doubly linked list with nodes 10 <-> 20.

1. Create new node with data 5.
2. Set new node's next to current head (10).
3. Set current head's previous to new node.
4. Update head to new node.
Result: 5 <-> 10 <-> 20

What pointer of the new node should be set when inserting at the beginning of a doubly linked list?

A Previous pointer to current head

B Next pointer to current head

C Next pointer to null

D Previous pointer to null

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

After inserting a new node at the beginning, what should the old head's previous pointer point to?

A The new node

B Null

C Itself

D The tail node

The old head's previous pointer must point back to the new node inserted at the beginning.

If the doubly linked list is empty, what happens when you insert a new node at the beginning?

A The new node becomes head and tail

B The new node is ignored

C The new node points to itself

D The list remains empty

When the list is empty, the new node is both the head and tail with no next or previous nodes.

Which pointer of the new node should be set to null when inserting at the beginning?

A Both next and previous pointers

B Next pointer

C Previous pointer

D Neither pointer

The new node's previous pointer should be null because it will be the first node.

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

A O(log n)

B O(n)

C O(n^2)

D O(1)

Insertion at the beginning is done by updating a few pointers, so it takes constant time O(1).

Explain step-by-step how to insert a new node at the beginning of a doubly linked list.

Think about pointers and the head 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 add a new node as the first element of the list

B. To remove the last node from the list

C. To search for a node in the list

D. To reverse the entire list

Solution

1. Step 1: Understand the operation

   Inserting at the beginning means adding a new node before the current first node.

2. Step 2: Identify the effect on the list

   The new node becomes the head (first node), shifting others to the right.

3. Final Answer:

   To add a new node as the first element of the list -> Option A

4. Quick Check:

   Insert at beginning = Add first node [OK]

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

Common Mistakes:

• Confusing insertion with deletion
• Thinking insertion adds at the end
• Mixing up search with insertion

2. Which of the following is the correct syntax to create a new node in C# for a doubly linked list?

easy

A. Node newNode = new Node(data);

B. newNode = Node(data);

C. Node newNode = Node(data);

D. Node newNode = new Node[];

Solution

1. Step 1: Recall C# object creation syntax

   In C#, objects are created using the 'new' keyword followed by the constructor call.

2. Step 2: Match syntax with node creation

   Correct syntax is 'Node newNode = new Node(data);' to create a new node with data.

3. Final Answer:

   Node newNode = new Node(data); -> Option A

4. Quick Check:

   C# new object = new ClassName() [OK]

Hint: Use 'new' keyword to create objects in C# [OK]

Common Mistakes:

• Omitting 'new' keyword
• Using array syntax for single node
• Missing type declaration

3. Consider this code snippet inserting a node with value 10 at the beginning of a doubly linked list:

Node newNode = new Node(10);
newNode.next = head;
if (head != null) {
    head.prev = newNode;
}
head = newNode;

What will be the printed list after insertion if the original list was: 20 <-> 30 <-> null?

medium

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

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

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

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

Solution

1. Step 1: Insert new node at beginning

   New node with 10 points next to old head (20). Old head's prev points back to new node.

2. Step 2: Update head pointer

   Head now points to new node (10), so list starts with 10.

3. Final Answer:

   10 <-> 20 <-> 30 <-> null -> Option D

4. Quick Check:

   New node at front = 10 first [OK]

Hint: New node points to old head; update old head's prev [OK]

Common Mistakes:

• Not updating old head's prev pointer
• Appending new node at end instead of beginning
• Not updating head to new node

4. Identify the error in this code snippet for inserting at the beginning of a doubly linked list:

Node newNode = new Node(5);
newNode.next = head;
head.prev = newNode;
head = newNode;

medium

A. Incorrect assignment of newNode.next

B. Missing check if head is null before accessing head.prev

C. head should be assigned before setting newNode.next

D. newNode.prev should be set to head

Solution

1. Step 1: Analyze head.prev assignment

   Code sets head.prev = newNode without checking if head is null, causing error if list is empty.

2. Step 2: Identify missing null check

   Must check if head != null before setting head.prev to avoid null reference error.

3. Final Answer:

   Missing check if head is null before accessing head.prev -> Option B

4. Quick Check:

   Check head != null before head.prev [OK]

Hint: Always check head != null before accessing head.prev [OK]

Common Mistakes:

• Ignoring null check on head
• Setting pointers in wrong order
• Confusing prev and next pointers

5. You have a doubly linked list with nodes: 5 <-> 15 <-> 25 <-> null. You want to insert a new node with value 1 at the beginning. After insertion, which of the following correctly represents the list and the pointers of the new node?

hard

A. 5 <-> 15 <-> 25 <-> 1 <-> null, newNode.prev = 25, newNode.next = null

B. 1 <-> 5 <-> 15 <-> 25 <-> null, newNode.prev = 5, newNode.next = null

C. 1 <-> 5 <-> 15 <-> 25 <-> null, newNode.prev = null, newNode.next = 5

D. 5 <-> 15 <-> 25 <-> 1 <-> null, newNode.prev = null, newNode.next = 5

Solution

1. Step 1: Insert new node at beginning

   New node with value 1 becomes head, points next to old head (5), prev is null.

2. Step 2: Confirm pointers and list order

   List order is 1 <-> 5 <-> 15 <-> 25 <-> null, newNode.prev = null, newNode.next = 5.

3. Final Answer:

   1 <-> 5 <-> 15 <-> 25 <-> null, newNode.prev = null, newNode.next = 5 -> Option C

4. Quick Check:

   New head prev=null, next=old head [OK]

Hint: New head prev=null, next=old head node [OK]

Common Mistakes:

• Setting newNode.prev to old head
• Appending new node at end instead of beginning
• Not updating head pointer