Linked List Operations in Python

Welcome to our tutorial focusing on Linked List Operations in Python. Singly-Linked Lists (or just Linked Lists) are one of the most fundamental data structures used in computer science. They provide an efficient way to store and access data that is not necessarily contiguous in memory. This ability separates linked lists from arrays, making them an indispensable tool in a programmer's toolkit.

To work with linked lists, we first need to define a ListNode class, which represents a node in the linked list.

class ListNode:
    def __init__(self, value=0, next=None):
        self.value = value     # Holds the value or data of the node
        self.next = next       # Points to the next node in the linked list; default is None

# Initialization of linked list
head = ListNode(1, ListNode(2, ListNode(3, ListNode(4, ListNode(5)))))

In the ListNode class:

• value holds the data of the node.
• next is a reference to the next node in the linked list. It is None by default, meaning the node does not point to any other node when it's freshly created.

To understand this, first know that a linked list is a linear data structure where each element is a separate object known as a node. A node comprises data and a reference (link) to the next node in the sequence.

The provided code creates a linked list where each node points to another as follows: 1 -> 2 -> 3 -> 4 -> 5, and the last node points to None.

One example of a problem to practice involves reversing a linked list, a common operation in interviews and industry. To reverse a linked list, we'll need to sequentially rearrange the next link of each node to point toward its previous node.

Here is what the code might look like. Note that this code uses $O(1)$ of additional memory:

class ListNode:
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next

def reverse_linked_list(head):
    prev = None
    current = head
    while current:
        next_node = current.next
        current.next = prev
        prev = current
        current = next_node
    return prev

# Test
head = ListNode(1, ListNode(2, ListNode(3, ListNode(4, ListNode(5)))))
reversed_head = reverse_linked_list(head)
while reversed_head:
    print(reversed_head.val, end=" ")
    reversed_head = reversed_head.next  # Output: 5 4 3 2 1