Insert at Beginning Head Insert in DSA Python - Build from Scratch

Insert at Beginning (Head Insert) in a Linked List

📖 Scenario: Imagine you are managing a line of people waiting to enter a concert. Sometimes, a VIP arrives and needs to be placed right at the front of the line. We will simulate this using a linked list where each person is a node.

🎯 Goal: You will build a simple linked list and learn how to insert a new node at the beginning (head) of the list. This means the new person will be first in line.

📋 What You'll Learn

Create a Node class with data and next attributes

Create a LinkedList class with a head attribute

Write a method insert_at_beginning to add a new node at the start

Print the linked list after insertion to show the order

💡 Why This Matters

🌍 Real World

Linked lists are used in many applications like music playlists, undo functionality in apps, and managing queues where order matters.

💼 Career

Understanding linked lists and how to insert nodes is fundamental for software developers, especially when working with dynamic data structures and memory management.

1

Create Node and LinkedList classes

Create a class called Node with an __init__ method that takes data and sets self.data = data and self.next = None. Then create a class called LinkedList with an __init__ method that sets self.head = None.

DSA Python

# Define Node class
# Define LinkedList class
# Your code here

Hint

Think of Node as a person in line holding data and a pointer to the next person. The LinkedList keeps track of the first person with head.

2

Add insert_at_beginning method

Inside the LinkedList class, define a method called insert_at_beginning that takes self and data. Create a new Node with data. Set the new node's next to the current head. Then update head to the new node.

DSA Python

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

class LinkedList:
    def __init__(self):
        self.head = None

    # Define insert_at_beginning method here
    # Your code here

Hint

Remember, the new node points to the old first node, then becomes the new first node.

3

Add method to print the linked list

Inside the LinkedList class, define a method called print_list that takes self. Use a variable current starting at head. Use a while loop to print current.data followed by ' -> ' and move current to current.next. After the loop, print None.

DSA Python

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

class LinkedList:
    def __init__(self):
        self.head = None

    def insert_at_beginning(self, data):
        new_node = Node(data)
        new_node.next = self.head
        self.head = new_node

    # Define print_list method here
    # Your code here

Hint

Traverse the list from head to end, printing each node's data followed by an arrow.

4

Insert nodes and print the list

Create a LinkedList object called ll. Use ll.insert_at_beginning(30), then ll.insert_at_beginning(20), then ll.insert_at_beginning(10). Finally, call ll.print_list() to display the list.

DSA Python

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

class LinkedList:
    def __init__(self):
        self.head = None

    def insert_at_beginning(self, data):
        new_node = Node(data)
        new_node.next = self.head
        self.head = new_node

    def print_list(self):
        current = self.head
        while current:
            print(str(current.data) + ' -> ', end='')
            current = current.next
        print('None')

# Create linked list and insert nodes
# Your code here

Hint

Insert 30 first, then 20 before it, then 10 before 20. The list should print from 10 to 30.