Insert at Beginning Head Insert in DSA Python - 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 the code snippet:
new_node.next = head
head = new_node
What is happening?

The new node's next pointer is set to the current head, then the head is updated to the new node, effectively inserting it at the beginning.

beginner

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

The old head becomes the second node because the new node points to it.

beginner

What is the printed output after inserting 3 nodes with values 10, 20, 30 at the beginning in that order?

30 -> 20 -> 10 -> null

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

A O(1)

B O(n)

C O(log n)

D O(n^2)

Inserting at the beginning only changes pointers and does not depend on list size, so it is O(1).

After inserting a new node at the beginning, what does the head pointer point to?

A The new node

B The last node

C The second node

D Null

The head pointer always points to the first node, which is the new node after insertion.

Which pointer of the new node is updated during insertion at the beginning?

A previous pointer

B head pointer

C next pointer

D tail pointer

The new node's next pointer is set to the old head node.

If the linked list is empty, what will the head point to after inserting a new node at the beginning?

A Null

B Old head

C Tail node

D The new node

When the list is empty, the new node becomes the head and points to null.

What will be the output after inserting nodes with values 5, 15, 25 at the beginning in that order?

A 5 -> 15 -> 25 -> null

B 25 -> 15 -> 5 -> null

C 15 -> 25 -> 5 -> null

D null -> 5 -> 15 -> 25

Each new node is inserted at the beginning, so the last inserted node (25) is first.

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

Think about what pointers need to change.

Describe how the linked list changes visually after inserting a node at the beginning.

Imagine a chain where you add a new link at the front.

Practice

1. What does the operation insert at beginning do in a linked list?

easy

A. Deletes the first node of the list

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

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

D. Deletes the last node of the list

Solution

1. Step 1: Understand the meaning of 'insert at beginning'

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

2. Step 2: Identify the effect on the linked list

   The new node becomes the new head, pushing the old head to second position.

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: Insert at beginning means new head node [OK]

Common Mistakes:

• Confusing insert at beginning with insert at end
• Thinking it deletes nodes instead of adding
• Assuming it changes the last node

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

easy

A. new_node = head.next; head = new_node

B. head.next = new_node; new_node = head

C. head = head.next; new_node = head

D. new_node.next = head; head = new_node

Solution

1. Step 1: Understand pointer assignment for head insert

   To insert at beginning, new_node's next points to current head.

2. Step 2: Update head to new_node

   Then head is updated to new_node to make it the first node.

3. Final Answer:

   new_node.next = head; head = new_node -> Option D

4. Quick Check:

   Set new_node.next to head, then head = new_node [OK]

Hint: Link new node to head, then update head [OK]

Common Mistakes:

• Assigning head.next instead of new_node.next
• Not updating head to new_node
• Swapping assignment order incorrectly

3. What will be the linked list after executing the following code?

class Node:
    def __init__(self, data):
        self.data = data
        self.next = None

head = Node(10)
new_node = Node(20)
new_node.next = head
head = new_node

medium

A. 20 -> 10 -> null

B. 10 -> 20 -> null

C. 20 -> null

D. 10 -> null

Solution

1. Step 1: Create initial list with head node 10

   Initially, head points to node with data 10 and next is None.

2. Step 2: Insert new_node with data 20 at beginning

   new_node.next points to head (node 10), then head updated to new_node.

3. Final Answer:

   20 -> 10 -> null -> Option A

4. Quick Check:

   New node 20 becomes head, points to 10 [OK]

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

Common Mistakes:

• Thinking new_node is added at end
• Not updating head to new_node
• Confusing node data order

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

class Node:
    def __init__(self, data):
        self.data = data
        self.next = None

head = Node(5)
new_node = Node(15)
head = new_node
new_node.next = head

medium

A. The order of assignments is wrong; new_node.next should be set before head update

B. Node class is missing a data attribute

C. head should be None initially

D. new_node should not be created before head

Solution

1. Step 1: Analyze assignment order

   head is set to new_node first, then new_node.next is set to head (which is now new_node itself).

2. Step 2: Identify logical error

   This causes new_node.next to point to itself, losing original list starting at old head.

3. Final Answer:

   The order of assignments is wrong; new_node.next should be set before head update -> Option A

4. Quick Check:

   Set new_node.next before head = new_node [OK]

Hint: Set new_node.next before updating head [OK]

Common Mistakes:

• Updating head before linking new_node.next
• Confusing attribute initialization
• Assuming head must be None initially

5. Given a linked list with nodes containing values [30, 20, 10] (30 is head), which sequence of operations correctly inserts a new node with value 40 at the beginning and results in the list 40 -> 30 -> 20 -> 10 -> null?

hard

A. new_node = Node(40); new_node.next = None; head = new_node

B. new_node = Node(40); head.next = new_node; head = new_node

C. new_node = Node(40); new_node.next = head; head = new_node

D. new_node = Node(40); head = new_node; new_node.next = head

Solution

1. Step 1: Create new node with value 40

   new_node is created with data 40 and next initially None.

2. Step 2: Link new_node to current head

   Set new_node.next to current head (node 30) to keep existing list after new node.

3. Step 3: Update head to new_node

   Head pointer is updated to new_node, making it the first node.

4. Final Answer:

   new_node = Node(40); new_node.next = head; head = new_node -> Option C

5. Quick Check:

   Link new node to old head, then update head [OK]

Hint: Link new node to head, then update head pointer [OK]

Common Mistakes:

• Linking head.next instead of new_node.next
• Updating head before linking new_node.next
• Setting new_node.next to None losing list