Alinkedlistisalineardatastructurewhereelementsarenotstorednexttoeachotherinmemory.Theelementsinalinkedlistare linkedusingpointers 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 wont do us any good. We need to examine the first elements 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, well build a Node class. The LinkedList class we eventually build will be a list of Nodes.
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 its None
Linked List Constructor
The constructor is easy just initialize an empty head pointer. This indicates we now have an empty list.
Iterating over the list
Lets make it easy to iterate over each item in the list using pythons for _ in _ syntax.
By implementing Pythons __iter__ method, we can now use iteration syntax. For example, for item in linked_list:.
Adding to the linked list
Lets 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.
Removing from the linked list
There are other ways to remove items from the list, but for now, and as an example, lets write a remove from head method.
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 Pythons __repr__[] method so that we can call print[] directly on a list and control what it printed. Heres a representation I like:
This method will print each nodes value in order, with arrows in between. For example, hello -> this -> is -> my -> list.
Using the linked list
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
| Insert | O[1] |
| Delete | O[1] |
| Index | O[n] |
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
Related Reading
- Writing a Binary Search Tree in Python