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Building a Linked List in Python with Examples

A linked list is a linear data structure where elements are not stored next to each other in memory. The elements in a linked list are linked using pointers or references. Linked lists are an ordered collection of objects, similar to a normal list. Linked lists stand apart from lists in how they store elements in memory. While regular lists like arrays and slices use a contiguous memory block to store references to their data, linked lists store references, aka pointers as part of each element.

A normal list is just a pointer to the first element in the list, and a specific item can be retrieved by providing a memory offset.

A linked list is also just a pointer to the first element in the list, but memory offsets won’t do us any good. We need to examine the first element’s next pointer to see where the next item is, then we can navigate to it. From there, we can find the next item and so on down the list.

Python singly linked list example

Node Class

First, we’ll build a Node class. The LinkedList class we eventually build will be a list of Nodes.

class Node:     def __init__(self, val):         self.val = val         self.next = None     def set_next(self, node):         self.next = node     def __repr__(self):         return self.val
Code language: Python (python)

Each node has a val data member (the information it stores) and a next data member. The next data member just points to the next Node in the list if there is one, otherwise it’s None

Linked List Constructor

class LinkedList:     def __init__(self):         self.head = None
Code language: Python (python)

The constructor is easy – just initialize an empty head pointer. This indicates we now have an empty list.

Iterating over the list

Let’s make it easy to iterate over each item in the list using python’s for _ in _ syntax.

def __iter__(self):         node = self.head         while node is not None:             yield node             node = node.next
Code language: Python (python)

By implementing Python’s __iter__ method, we can now use iteration syntax. For example, for item in linked_list:.

Adding to the linked list

Let’s create a way to add items to the tail of the list, the add_to_tail method. It takes a node as input, iterates over the entire list, then adds the given node to the end.

def add_to_tail(self, node):         if self.head == None:             self.head = node             return         for current_node in self:             pass         current_node.set_next(node)
Code language: Python (python)

Removing from the linked list

There are other ways to remove items from the list, but for now, and as an example, let’s write a remove from head method.

def remove_from_head(self):         if self.head == None:             return None         temp = self.head         self.head = self.head.next         return temp
Code language: Python (python)

remove_from_head removes and returns the first item from the list, assuming one exists.

Printing the linked list

Last but not least, we can implement Python’s __repr__() method so that we can call print() directly on a list and control what it printed. Here’s a representation I like:

def __repr__(self):         nodes = []         for node in self:             nodes.append(node.val)         return " -> ".join(nodes)
Code language: Python (python)

This method will print each node’s value in order, with arrows in between. For example, hello -> this -> is -> my -> list.

Using the linked list

linked_list = LinkedList() linked_list.add_to_tail(Node('john')) linked_list.add_to_tail(Node('sally')) linked_list.add_to_tail(Node('jimmy')) print("ll:", linked_list) first = linked_list.remove_from_head() print("removed:", first) print("ll:", linked_list)
Code language: Python (python)

Practical Applications of a Linked List

Linked lists are immensely valuable in computer science because they uniquely allow us to add and remove elements anywhere in the list quickly, with a Big-O complexity of just O(1).

Big-O complexity of a linked list

OperationBig-O Complexity

Because of the fast operations, linked lists are used practically in many different scenarios, including:

  • Stacks
  • Queues
  • Hash maps, to prevent collisions
  • Undo/Redo operations (stack)
  • Appending a song to a playlist
  • To keep items in the same place in memory for performance reasons

Full Linked List Code Sample

class LinkedList:     def __init__(self):         self.head = None     def __iter__(self):         node = self.head         while node is not None:             yield node             node = node.next     def __repr__(self):         nodes = []         for node in self:             nodes.append(node.val)         return " -> ".join(nodes)     def add_to_tail(self, node):         if self.head == None:             self.head = node             return         for current_node in self:             pass         current_node.set_next(node)     def remove_from_head(self):         if self.head == None:             return None         temp = self.head         self.head = self.head.next         return temp class Node:     def __init__(self, val):         self.val = val         self.next = None     def set_next(self, node):         self.next = node     def __repr__(self):         return self.val
Code language: Python (python)

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