Linked Lists

Note

Source: Adapted from the C# edition (datastructures/datastructures.rst). Python uses None where C# uses null as the end-of-list sentinel. The Node class and linked-list logic are otherwise directly analogous to the C# pointer-based implementation.

A linked list stores elements in nodes, where each node holds a value and a reference to the next node. Unlike Python’s built-in list, a linked list does not require a contiguous block of memory.

The Node Class

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

Each node points to the next node in the chain. The last node’s next is None — the Python equivalent of a null pointer.

Building a Linked List by Hand

# Build the list  1 -> 2 -> 3 -> None
head = Node(1, Node(2, Node(3)))

Traversal

Walk the list by following next until None:

def print_list(head):
    current = head
    while current is not None:
        print(current.data, end=" -> ")
        current = current.next
    print("None")

print_list(head)

Output:

1 -> 2 -> 3 -> None

The SinglyLinkedList Class

A wrapper class manages the head pointer and provides clean methods:

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

    def prepend(self, data):
        self.head = Node(data, self.head)

    def append(self, data):
        new_node = Node(data)
        if self.head is None:
            self.head = new_node
            return
        current = self.head
        while current.next is not None:
            current = current.next
        current.next = new_node

    def remove(self, data):
        if self.head is None:
            return
        if self.head.data == data:
            self.head = self.head.next
            return
        current = self.head
        while current.next is not None:
            if current.next.data == data:
                current.next = current.next.next
                return
            current = current.next

    def __iter__(self):
        current = self.head
        while current is not None:
            yield current.data
            current = current.next

    def __str__(self):
        return " -> ".join(str(x) for x in self) + " -> None"

lst = SinglyLinkedList()
lst.append(10)
lst.append(20)
lst.append(30)
lst.prepend(5)
print(lst)

lst.remove(20)
print(lst)

Output:

5 -> 10 -> 20 -> 30 -> None
5 -> 10 -> 30 -> None

Performance Comparison

Operation	List	LinkedList
Index access	O(1)	O(N)
Prepend	O(N)	O(1)
Append	O(1) amort	O(N)
Insert in middle	O(N)	O(N)
Remove from front	O(N)	O(1)

Python’s built-in list is a dynamic array and is faster for most uses. Linked lists shine when you need O(1) prepend or frequent insertions at a known position.