Data Structures - Linked Lists

Before we dive into a new data structure, be aware that there is a great quick guide on Computer Science concepts (data structures and common algorithms) that you can access here.

Now, let's discuss another data structure that builds off of our knowledge of arrays: the Linked List.

The linked list is made up of units called nodes, and each node contains two things: a value, and a pointer to the next node so that we can find it in memory. Unlike arrays, linked lists are collections of data stored that are not stored contiguously. Because of this, linked lists can grow in size very easily; you can simply add on an object to a linked list anywhere in memory, and the previous node will simply contain a pointer to the new node, wherever it may be.

Linked lists, as data structures, are often used to implement other abstract data types; they describe functionality more than implementation. The underlying implementation can be due to data structures such as arrays, hash tables, linked lists, etc. For example, stacks, queues, and associate arrays can be implemented using linked lists; they themselves are abstract data types, but are implemented using data structures.

Another way to differentiate between data structures and abstract data types is that data structures are looked at from the perspective of the person implementing them; an abstract data type is viewed from the perspective of a user, meaning that the data is described by its behavior.

Just as a developer can have variations of a standard array (think dimensions), we can have doubly linked lists, where nodes contain pointers to both the next node and the previous node, and even circular linked lists, where instead of the last node pointing to a "dummy" node, as is sometimes done, the last node contains a pointer to the first one. Linked lists support sequential access; that is, you must traverse from the "head" node through the list until you find your desired object. This means that indexing in a linked list is a linear time function; the Big O notation for linked list access (and also searching) is O(n).

                                This is a diagram illustrating the insertion of a node to an existing linked list


This is a diagram illustrating the insertion of a node to an existing linked list


However, insertion and deletion are optimized operations in linked lists - they are both constant time functions (O(1)). If something needs to be inserted, we simply redirect the pointers in the linked list so that the intended previous one points to the new node, and the new node points to the intended next one. The same goes for deletion; we can simply redirect the node pointers so that nothing points to a certain node, and deallocate it from memory.

*note - if you don't already have a pointer to a node that you want to delete, the actual time for deletion would be (search_time) + O(1). The O(1) represents the constant time deletion. The same goes for insertion.

Arrays, at least in my experience, are used far more often than linked lists. There isn't even support for linked lists in Objective-C, although you can implement your own in one of a few ways; check stack overflow for more information on replicating linked list functionality in Objective-C.

One time a linked list may be useful is when using it to implement, for example, a queue. As we will learn, queues are FIFO (first in, last out), which means that the first object to be placed on a queue is the first one to exit the queue. Because insertion is a constant time function for linked lists, inserting to and deletion from the middle of a linked list is optimized. For a priority queue, in which you may break the FIFO rule when something has a high priority flag placed on it, linked lists would work nicely since you can remove directly from themiddle of one.

Next time we'll be covering very simple abstract data types - stacks and queues!