Linked List Introduced

Linked lists are a fundamental data structure used in computer science to organize and store data in a linear sequence, where each element points to the next, making it easy to insert or delete elements.

The Big Picture

Imagine a linked list as a chain of paper clips. Each paper clip is a node, and the way they are linked together forms the chain. Unlike an array where elements are stored in contiguous memory locations, each node in a linked list is separate and contains a reference (or link) to the next node in the sequence.

Core Concepts

1. Node: The basic unit of a linked list, which contains data and a reference (or pointer) to the next node.
2. Head: The first node in the linked list. If the list is empty, the head is null.
3. Pointer/Reference: The link part of a node that points to the next node in the list.
4. Null Reference: The end of the list is indicated by a node that has a null reference, meaning it points to nothing.

Detailed Walkthrough

1. Node Structure:

   • Each node typically has two parts: the data it holds and a pointer to the next node.
   • In a singly linked list, each node points only to the next node.
   • In a doubly linked list, each node has two pointers: one to the next node and one to the previous node.

2. Creating a Linked List:

   • Start with the head node.
   • Each new node is added by adjusting the pointers.
   • For example, to add a node to the end, find the last node and update its pointer to the new node.

3. Operations:

   • Insertion: Adding a new node to the list, which involves changing pointers.
     • At the beginning: Adjust the head to the new node.
     • At the end: Find the last node and update its pointer.
     • In the middle: Adjust the pointers of the surrounding nodes.
   • Deletion: Removing a node, which involves bypassing the node to be removed and adjusting the pointers.
   • Traversal: Accessing each node sequentially from the head to the end to perform operations like searching or printing the list.

Understanding Through an Example

Let's consider a simple singly linked list with the nodes containing integers.

1. Creating the List:

   class Node:
       def __init__(self, data):
           self.data = data
           self.next = None
   
   class LinkedList:
       def __init__(self):
           self.head = None
   
       def append(self, data):
           new_node = Node(data)
           if not self.head:
               self.head = new_node
               return
           last = self.head
           while last.next:
               last = last.next
           last.next = new_node
   
       def print_list(self):
           current = self.head
           while current:
               print(current.data)
               current = current.next
   
   # Example Usage
   ll = LinkedList()
   ll.append(1)
   ll.append(2)
   ll.append(3)
   ll.print_list()

   In this example:

   • We define a Node class with data and next attributes.
   • We define a LinkedList class with methods to append nodes and print the list.
   • We create a linked list and append three nodes with values 1, 2, and 3.

2. Traversing the List:

   • The print_list method iterates through the nodes starting from the head, printing each node's data.

Conclusion and Summary

Linked lists are a flexible and dynamic data structure that allows for efficient insertion and deletion of elements. Each node contains data and a pointer to the next node, forming a chain-like structure. This structure is particularly useful in scenarios where the size of the list can change frequently, and memory allocation needs to be dynamic.

Test Your Understanding

1. What is the difference between a singly linked list and a doubly linked list?
2. How would you modify the LinkedList class to include a method that deletes a node with a specific value?
3. Can you explain how you would find the middle element of a linked list?

Reference

• GeeksforGeeks: Linked List Data Structure
• Wikipedia: Linked List

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