Insert at Beginning Head Insert in DSA C# - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does 'Insert at Beginning' mean in a linked list?

It means adding a new node at the start (head) of the linked list, making it the first element.

beginner

Why is inserting at the beginning of a linked list efficient?

Because it only requires changing the head pointer and the new node's next pointer, so it takes constant time O(1).

beginner

In C#, what is the first step when inserting a new node at the beginning of a singly linked list?

Create a new node with the given data and set its next pointer to the current head node.

beginner

What happens to the head pointer after inserting a new node at the beginning?

The head pointer is updated to point to the new node, making it the first node in the list.

beginner

Show the output of the linked list after inserting 3, then 5 at the beginning.

After inserting 3: 3 -> null
After inserting 5: 5 -> 3 -> null

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

A O(log n)

B O(n)

C O(n^2)

D O(1)

Inserting at the beginning only changes pointers and does not require traversal, so it takes constant time O(1).

When inserting at the beginning, what does the new node's next pointer point to?

A Null

B The current head node

C The last node

D Itself

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

After inserting a new node at the beginning, what should the head pointer reference?

A The old head node

B Null

C The new node

D The last node

The head pointer must be updated to the new node to reflect the new start of the list.

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

A Null

B Itself

C The old head

D The last node

If the list is empty, the new node's next pointer should be null because there are no other nodes.

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

A Traverse the entire list

B Create a new node

C Set new node's next to current head

D Update head to new node

Traversal is not needed when inserting at the beginning; only pointer updates are required.

Explain how to insert a new node at the beginning of a singly linked list.

Think about how the head pointer changes and how the new node links to the old list.

Describe the state of the linked list after inserting nodes 10, then 20 at the beginning.

Remember the order is reversed because each new node becomes the head.

Practice

1. What does the "Insert at Beginning" operation do in a singly linked list?

easy

A. Deletes the last node of the list

B. Adds a new node at the start of the list

C. Deletes the first node of the list

D. Adds a new node at the end of the list

Solution

1. Step 1: Understand the operation

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

2. Step 2: Effect on the list

   The new node becomes the head, pushing existing nodes after it.

3. Final Answer:

   Adds a new node at the start of the list -> Option B

4. Quick Check:

   Insert at beginning = Add new head [OK]

Hint: New node becomes head, old head moves next [OK]

Common Mistakes:

• Confusing insert at beginning with insert at end
• Thinking it deletes nodes
• Assuming it changes the tail node

2. Which of the following is the correct syntax to insert a new node at the beginning of a singly linked list in C#?

easy

A. new Node(data).next = head; head = new Node(data);

B. head = new Node(data); head.next = oldHead;

C. head.next = new Node(data);

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

Solution

1. Step 1: Create new node and link

   Create a new node and set its next pointer to current head.

2. Step 2: Update head pointer

   Assign head to the new node to make it the first node.

3. Final Answer:

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

4. Quick Check:

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

Hint: New node next = head; head = new node [OK]

Common Mistakes:

• Not updating head after creating new node
• Assigning head before setting newNode.next
• Creating multiple new nodes unnecessarily

3. What will be the output of the following code snippet?

class Node {
  public int data;
  public Node next;
  public Node(int d) { data = d; next = null; }
}

Node head = new Node(10);
head.next = new Node(20);
Node newNode = new Node(5);
newNode.next = head;
head = newNode;

Node temp = head;
while(temp != null) {
  Console.Write(temp.data + " -> ");
  temp = temp.next;
}
Console.WriteLine("null");

medium

A. 5 -> 10 -> 20 -> null

B. 20 -> 10 -> 5 -> null

C. 10 -> 20 -> 5 -> null

D. 5 -> 20 -> 10 -> null

Solution

1. Step 1: Initial list creation

   Head points to 10, which points to 20 (10 -> 20 -> null).

2. Step 2: Insert new node at beginning

   New node with data 5 points to old head (10), then head updated to new node (5).

3. Step 3: Traverse and print list

   Traversal prints 5 -> 10 -> 20 -> null.

4. Final Answer:

   5 -> 10 -> 20 -> null -> Option A

5. Quick Check:

   New head 5, followed by 10 and 20 [OK]

Hint: New node points to old head, becomes new head [OK]

Common Mistakes:

• Confusing order of nodes after insertion
• Not updating head pointer
• Printing nodes before insertion

4. Identify the error in the following code that tries to insert a node at the beginning of a singly linked list:

Node newNode = new Node(15);
newNode.next = newNode;
head = newNode;

medium

A. head should be assigned before setting newNode.next

B. Node constructor is missing data parameter

C. newNode.next should point to head, not newNode

D. No error, code is correct

Solution

1. Step 1: Analyze newNode.next assignment

   newNode.next points to itself, creating a cycle instead of linking to the list.

2. Step 2: Correct pointer assignment

   newNode.next should point to current head to insert at beginning properly.

3. Final Answer:

   newNode.next should point to head, not newNode -> Option C

4. Quick Check:

   Link new node to old head, not itself [OK]

Hint: newNode.next = head, not newNode [OK]

Common Mistakes:

• Setting next pointer to newNode itself
• Not updating head after insertion
• Confusing order of assignments

5. You have a singly linked list with nodes containing values [30, 40, 50]. You want to insert nodes with values [10, 20] at the beginning in that order so the final list is [10, 20, 30, 40, 50]. Which sequence of insertions achieves this?

hard

A. Insert 20 at beginning, then insert 10 at beginning

B. Insert 10 at beginning, then insert 20 at beginning

C. Insert 10 at end, then insert 20 at beginning

D. Insert 20 at end, then insert 10 at beginning

Solution

1. Step 1: Understand insertion order effect

   Each insert at beginning places new node before current head.

2. Step 2: Insert 10 first, then 20

   Inserting 10 first results in list [10, 30, 40, 50]. Then inserting 20 at beginning results in [20, 10, 30, 40, 50].

3. Step 3: Reverse the order to get desired list

   To get [10, 20, 30, 40, 50], insert 20 first, then 10.

4. Final Answer:

   Insert 20 at beginning, then insert 10 at beginning -> Option A

5. Quick Check:

   Last inserted node becomes new head [OK]

Hint: Insert last value first to keep order at head [OK]

Common Mistakes:

• Inserting in given order causes reversed nodes
• Confusing insert at beginning with insert at end
• Not updating head after each insertion