Project: Linked Lists

Ruby Course


In Computer Science, one of the most basic and fundamental data structures is the linked list, which functions similarly to an array. The principal benefit of a linked list over a conventional array is that the list elements can easily be inserted or removed without reallocation of any other elements.

In some programming languages, the size of an array is a concern and one of the ways to overcome that problem and allow dynamically allocated data is using linked lists.

Luckily in Ruby, arrays aren’t limited to a certain size, and both insertion and deletion can be done trivially at any index using the appropriate built in array method, so you don’t have to think about overcoming those limitations.

So if array size, array insertion and array deletion are not limitations in Ruby, are linked lists really necessary? The short answer to that is no; however, it’s the simplest of the dynamic data structures and it will give you a solid foundation, so you can understand more complex data structures like graphs and binary trees with more ease.

Structure of a linked list

A linked list is a linear collection of data elements called nodes that “point” to the next node by means of a pointer.

Each node holds a single element of data and a link or pointer to the next node in the list.

A head node is the first node in the list, a tail node is the last node in the list. Below is a basic representation of a linked list:

[ NODE(head) ] -> [ NODE ] -> [ NODE(tail) ] -> nil

For a more thorough explanation, use these resources:

  1. Linked Lists in Plain English
  2. What’s a Linked List, Anyway?
  3. Linked Lists, Ruby’s Missing Data Structure
  4. A more verbose explanation with plenty of diagrams


You will need two classes:

  1. LinkedList class, which will represent the full list.
  2. Node class, containing a #value method and a link to the #next_node, set both as nil by default.

Build the following methods in your linked list class:

  1. #append(value) adds a new node containing value to the end of the list
  2. #prepend(value) adds a new node containing value to the start of the list
  3. #size returns the total number of nodes in the list
  4. #head returns the first node in the list
  5. #tail returns the last node in the list
  6. #at(index) returns the node at the given index
  7. #pop removes the last element from the list
  8. #contains?(value) returns true if the passed in value is in the list and otherwise returns false.
  9. #find(value) returns the index of the node containing value, or nil if not found.
  10. #to_s represent your LinkedList objects as strings, so you can print them out and preview them in the console. The format should be: ( value ) -> ( value ) -> ( value ) -> nil

Extra credit

  1. #insert_at(value, index) that inserts a new node with the provided value at the given index.
  2. #remove_at(index) that removes the node at the given index.

Extra Credit Tip: When you insert or remove a node, consider how it will affect the existing nodes. Some of the nodes will need their #next_node link updated.

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