Implement an singly-linked list in Ruby
The linked list is a fundamental data structure in computer science, often used in the implementation of other data structures. They're pervasive in functional programming languages, such as Clojure, Erlang, or Haskell, but far less common in imperative languages such as Ruby or Python.
The simplest kind of linked list is an immutable singly-linked list, which is the kind that's built-in to these functional programming languages. Immutable (or more specifically: persistent) data structures can be copied in constant time (since a reference is equivalent to a copy), and can save loads of time and memory for certain use cases because structure can be shared between versions of the data structure.
This variant of linked lists is often used to represent sequences or push-down stacks (also called a LIFO stack; Last In, First Out), a data structure that can be visualized like a can of Pringles. You can only access the topmost element in the stack, and it's always the most recently added.
Constant Time - when operation time is independent of problem size, that operation is a constant time (or O(1) time) operation. Accessing an element at a given index of an array is a constant time operation, because no matter how big an array is, it doesn't affect the time required to access an element by position. Finding the maximum element of an array, on the other hand, is not a constant time operation, because a full scan of the elements is required and the time required to complete that scan grows with the number of elements.
As a first take, lets create a persistent singly-linked list with just Node objects containing the range (1..10), and provide functions to reverse a linked list and convert to and from arrays. Use the provided rspec tests in spec/simple_linked_list_spec.rb to guide development.
When implementing this in a language with built-in linked lists, implement your own abstract data type.
We want to create a Node class that represents a node in a singly-linked list. An Node only understands two things: data - the piece of information that Node holds, and next, which is a reference to the next item in the list.
Day-to-Day users won't be interacting with our Node class, they will be interacting with a new layer of abstraction housed in a LinkedList class. Your LinkedList class should have the following methods. Remember NO USING ARRAYS.
#index_atshould take an index in and return the data at that indexinsert_at_indexshould take an index and data, and insert that data at that index shifting everything else downunshiftshould remove something from the head of the linked listpushshould add one piece of data do the end of the Linked List
Finally, we want to be able to reverse the linked list.
You'll be writing a #reverse method as well as the required tests to make sure it works!
If we envision the list as a piece of data (datum) and a pointer to the next element, then reversing a list is a matter of reversing the pointers while retaining the data.
If we have a list with elements A,B,C,D that looks like this:
A -> B -> C -> D, then reversing that list at element A would make it this:
D -> C -> B -> A. Essentially, the next of each element is reversed, so where D used to have a next of nil, it now has a next of C. Where A once had a next of B, it now has a next of nil.
Another way to visualize it would be like this: A <- B <- C <- D. Keep in mind that we are still implementing only next in this list, even though the directionality appears to indicate a previous style relationship. In a singly-linked list, there is only one direction.
Think about the different trade offs and ways to handle this reverse method. Some use more memory, and less time...others use more time and less memory!
Some more general reading about linked lists:
http://en.wikipedia.org/wiki/Linked_list http://cjlwired.github.io/blog/2013/08/08/linked-lists-and-ruby/ http://en.wikipedia.org/wiki/Singly_linked_list#Singly_linked_lists http://en.wikipedia.org/wiki/Doubly_linked_list
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