Insert at Beginning of Circular Linked List in DSA Python - Build from Scratch

Insert at Beginning of Circular Linked List

📖 Scenario: You are managing a circular linked list that represents a round-robin queue of tasks. You need to add a new task at the beginning of the queue so it gets priority.

🎯 Goal: Build a circular linked list and write code to insert a new node at the beginning, maintaining the circular structure.

📋 What You'll Learn

Create a Node class with data and next attributes

Create a circular linked list with three nodes containing data 10, 20, and 30

Create a function insert_at_beginning(head, data) that inserts a new node at the start

Print the circular linked list starting from the head after insertion

💡 Why This Matters

🌍 Real World

Circular linked lists are used in real-world systems like task schedulers, multiplayer games turn management, and buffering data streams where the list loops back to the start.

💼 Career

Understanding circular linked lists and insertion operations is important for software engineers working on system programming, embedded systems, and applications requiring efficient cyclic data management.

1

Create the Circular Linked List with 3 Nodes

Create a class called Node with attributes data and next. Then create three nodes with data 10, 20, and 30. Link them in a circular way so that the last node points back to the first node. Assign the first node to a variable called head.

DSA Python

# Define Node class and create circular linked list with nodes 10, 20, 30
# Your code here

Hint

Remember to make the last node point back to the first node to form a circle.

2

Create a Function to Insert at Beginning

Create a function called insert_at_beginning(head, data) that creates a new node with the given data and inserts it at the beginning of the circular linked list. The function should return the new head of the list.

DSA Python

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

node1 = Node(10)
node2 = Node(20)
node3 = Node(30)

node1.next = node2
node2.next = node3
node3.next = node1

head = node1

# Define insert_at_beginning function below
# Your code here

Hint

Traverse to the last node to update its next pointer to the new node.

3

Insert a New Node with Data 5 at Beginning

Use the function insert_at_beginning(head, 5) to insert a new node with data 5 at the beginning of the circular linked list. Update the head variable to the returned new head.

DSA Python

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

node1 = Node(10)
node2 = Node(20)
node3 = Node(30)

node1.next = node2
node2.next = node3
node3.next = node1

head = node1

def insert_at_beginning(head, data):
    new_node = Node(data)
    if head is None:
        new_node.next = new_node
        return new_node
    else:
        temp = head
        while temp.next != head:
            temp = temp.next
        temp.next = new_node
        new_node.next = head
        return new_node

# Insert new node with data 5 at beginning and update head
# Your code here

Hint

Call the function and assign its result back to head.

4

Print the Circular Linked List Starting from Head

Write code to print the circular linked list starting from head. Print each node's data followed by ->. Stop printing when you reach the head again. End the output with null.

DSA Python

Hint

Use a while True loop and break when you reach the head again.

Practice

1. What happens when you insert a new node at the beginning of a circular linked list?

easy

A. The new node replaces the last node in the list.

B. The new node becomes the head and points to the old head, maintaining the circular link.

C. The list breaks and becomes a linear linked list.

D. The new node is added at the end and does not affect the head.

Solution

1. Step 1: Understand the circular linked list structure

   A circular linked list has its last node pointing back to the head node, forming a circle.

2. Step 2: Insert new node at beginning

   The new node is linked to the current head, and the last node's next pointer is updated to this new node, making it the new head.

3. Final Answer:

   The new node becomes the head and points to the old head, maintaining the circular link. -> Option B

4. Quick Check:

   New head points to old head, last node points to new head [OK]

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

Common Mistakes:

• Forgetting to update last node's next pointer
• Breaking the circular link by not linking last node
• Replacing the last node instead of adding new head

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

easy

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

B. new_node = Node(data); last.next = head; head = new_node

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

D. new_node = Node(data); new_node.next = head; last.next = new_node; head = new_node

Solution

1. Step 1: Create new node and link to current head

   We create a new node and set its next pointer to the current head.

2. Step 2: Update last node's next pointer and head

   Since it's circular, last node's next must point to new node, then update head to new node.

3. Final Answer:

   new_node = Node(data); new_node.next = head; last.next = new_node; head = new_node -> Option D

4. Quick Check:

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

Hint: Update last.next to new node before changing head [OK]

Common Mistakes:

• Not updating last.next to new node
• Changing head before updating last.next
• Linking new node incorrectly

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

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

def print_list(head):
    temp = head
    if head is not None:
        while True:
            print(temp.data, end=' -> ')
            temp = temp.next
            if temp == head:
                break
    print('null')

# Initial circular linked list: 20 -> 30 -> 40 -> null
head = Node(20)
second = Node(30)
third = Node(40)
head.next = second
second.next = third
third.next = head

# Insert 10 at beginning
new_node = Node(10)
new_node.next = head
third.next = new_node
head = new_node

print_list(head)

medium

A. 10 -> 30 -> 40 -> 20 -> null

B. 20 -> 30 -> 40 -> 10 -> null

C. 10 -> 20 -> 30 -> 40 -> null

D. 30 -> 40 -> 20 -> 10 -> null

Solution

1. Step 1: Understand the initial list and insertion

   The list is 20 -> 30 -> 40 -> back to 20. Inserting 10 at beginning means new_node points to old head (20), and last node (40) points to new_node.

2. Step 2: Trace the print_list function

   Starting from new head (10), it prints 10, then 20, then 30, then 40, then stops when it reaches head again.

3. Final Answer:

   10 -> 20 -> 30 -> 40 -> null -> Option C

4. Quick Check:

   New head 10 followed by old nodes in order [OK]

Hint: New head prints first, then old nodes in order [OK]

Common Mistakes:

• Not updating last node's next pointer
• Printing from old head instead of new head
• Confusing order of nodes after insertion

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

def insert_begin(head, data):
    new_node = Node(data)
    new_node.next = head
    head = new_node
    return head

medium

A. The last node's next pointer is not updated to new_node, breaking the circular link.

B. The new_node.next should point to null instead of head.

C. The head should not be updated to new_node.

D. The function should not return head.

Solution

1. Step 1: Check pointer updates for circular list

   In a circular linked list, the last node's next pointer must point to the head node to maintain the circle.

2. Step 2: Identify missing update

   The code updates new_node.next and head but does not update the last node's next pointer to new_node, breaking the circle.

3. Final Answer:

   The last node's next pointer is not updated to new_node, breaking the circular link. -> Option A

4. Quick Check:

   Last node must point to new head to keep circle [OK]

Hint: Always update last.next to new head in circular list [OK]

Common Mistakes:

• Ignoring last node pointer update
• Setting new_node.next to null
• Not returning updated head

5. Given a circular linked list with nodes 5 -> 15 -> 25 -> back to 5, write the steps to insert a new node with value 1 at the beginning and then print the list. What will be the printed output?

hard

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

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

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

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

Solution

1. Step 1: Insert new node at beginning

   Create new node with value 1, set its next to current head (5), update last node (25) next to new node, then update head to new node.

2. Step 2: Print the circular linked list

   Start from new head (1), print nodes in order: 1, 5, 15, 25, then stop when back to head.

3. Final Answer:

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

4. Quick Check:

   New head 1 followed by old nodes in order [OK]

Hint: Update last.next before changing head to new node [OK]

Common Mistakes:

• Not updating last node's next pointer
• Printing from old head instead of new head
• Confusing node order after insertion