angelikatyborska's solution to Binary Search Tree on the Ruby track

Instructions

Test suite

Solution

Note:

This solution was written on an old version of Exercism. The tests below might not correspond to the solution code, and the exercise may have changed since this code was written.

Insert and search for numbers in a binary tree.

When we need to represent sorted data, an array does not make a good data structure.

Say we have the array [1, 3, 4, 5], and we add 2 to it so it becomes [1, 3, 4, 5, 2] now we must sort the entire array again! We can improve on this by realizing that we only need to make space for the new item [1, nil, 3, 4, 5], and then adding the item in the space we added. But this still requires us to shift many elements down by one.

Binary Search Trees, however, can operate on sorted data much more efficiently.

A binary search tree consists of a series of connected nodes. Each node contains a piece of data (e.g. the number 3), a variable named left, and a variable named right. The left and right variables point at nil, or other nodes. Since these other nodes in turn have other nodes beneath them, we say that the left and right variables are pointing at subtrees. All data in the left subtree is less than or equal to the current node's data, and all data in the right subtree is greater than the current node's data.

For example, if we had a node containing the data 4, and we added the data 2, our tree would look like this:

  4
 /
2

If we then added 6, it would look like this:

  4
 / \
2   6

If we then added 3, it would look like this

And if we then added 1, 5, and 7, it would look like this

      4
    /   \
   /     \
  2       6
 / \     / \
1   3   5   7

For installation and learning resources, refer to the exercism help page.

For running the tests provided, you will need the Minitest gem. Open a terminal window and run the following command to install minitest:

gem install minitest

If you would like color output, you can require 'minitest/pride' in the test file, or note the alternative instruction, below, for running the test file.

Run the tests from the exercise directory using the following command:

ruby binary_search_tree_test.rb

To include color from the command line:

ruby -r minitest/pride binary_search_tree_test.rb

Source

Josh Cheek https://twitter.com/josh_cheek

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

binary_search_tree_test.rb

require 'minitest/autorun'
require_relative 'binary_search_tree'

class BstTest < Minitest::Test
  def test_data_is_retained
    assert_equal 4, Bst.new(4).data
  end

  def test_inserting_less
    skip
    four = Bst.new 4
    four.insert 2
    assert_equal 4, four.data
    assert_equal 2, four.left.data
  end

  def test_inserting_same
    skip
    four = Bst.new 4
    four.insert 4
    assert_equal 4, four.data
    assert_equal 4, four.left.data
  end

  def test_inserting_right
    skip
    four = Bst.new 4
    four.insert 5
    assert_equal 4, four.data
    assert_equal 5, four.right.data
  end

  def test_complex_tree
    skip
    four = Bst.new 4
    four.insert 2
    four.insert 6
    four.insert 1
    four.insert 3
    four.insert 7
    four.insert 5
    assert_equal 4, four.data
    assert_equal 2, four.left.data
    assert_equal 1, four.left.left.data
    assert_equal 3, four.left.right.data
    assert_equal 6, four.right.data
    assert_equal 5, four.right.left.data
    assert_equal 7, four.right.right.data
  end

  def record_all_data(bst)
    all_data = []
    bst.each { |data| all_data << data }
    all_data
  end

  def test_iterating_one_element
    skip
    assert_equal [4], record_all_data(Bst.new(4))
  end

  def test_iterating_over_smaller_element
    skip
    four = Bst.new 4
    four.insert 2
    assert_equal [2, 4], record_all_data(four)
  end

  def test_iterating_over_larger_element
    skip
    four = Bst.new 4
    four.insert 5
    assert_equal [4, 5], record_all_data(four)
  end

  def test_iterating_over_complex_tree
    skip
    four = Bst.new 4
    four.insert 2
    four.insert 1
    four.insert 3
    four.insert 6
    four.insert 7
    four.insert 5
    assert_equal [1, 2, 3, 4, 5, 6, 7], record_all_data(four)
  end

  def test_each_returns_enumerator_if_no_block
    skip

    tree = Bst.new 4
    [2, 1, 3, 6, 7, 5].each { |x| tree.insert x }
    each_enumerator = tree.each

    assert_kind_of Enumerator, each_enumerator

    (1..7).each { |x| assert_equal(x, each_enumerator.next) }

    assert_raises(StopIteration) { each_enumerator.next }
  end

  # Problems in exercism evolve over time, as we find better ways to ask
  # questions.
  # The version number refers to the version of the problem you solved,
  # not your solution.
  #
  # Define a constant named VERSION inside of the top level BookKeeping
  # module, which may be placed near the end of your file.
  #
  # In your file, it will look like this:
  #
  # module BookKeeping
  #   VERSION = 1 # Where the version number matches the one in the test.
  # end
  #
  # If you are curious, read more about constants on RubyDoc:
  # http://ruby-doc.org/docs/ruby-doc-bundle/UsersGuide/rg/constants.html

  def test_bookkeeping
    skip
    assert_equal 1, BookKeeping::VERSION
  end
end

class Bst
  attr_reader :data, :left, :right

  def initialize(data)
    @data = data
    @left = nil
    @right = nil
  end

  def insert(data)
    if data <= @data
      @left.nil? ? @left = self.class.new(data) : @left.insert(data)
    else
      @right.nil? ? @right = self.class.new(data) : @right.insert(data)
    end
  end

  def each(&block)
    @left.each(&block) unless @left.nil?

    yield @data

    @right.each(&block) unless @right.nil?
  end
end

Community comments

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angelikatyborska

Solution Author

commented about 5 years ago

With advanced refactoring tools we all (hopefully) use it probably doesn't matter that much, but I don't like relying on explicit class names when I don't have to.

When I write self.class.new, what I really mean is "give me a new object of the class whose instance method I'm currently defining, doesn't matter if its called Bst or Banana, it only matters that it's the same class'. That's the case here - what matters the most is that a left and a right subtree of a tree is still a tree. On the other hand, when I write Bst.new, I mean "give me a new object of the class Bst, doesn't matter where I am now, it has to be Bst`.

Let's say someone wanted to rename that class and didn't use any refactoring tools. They would have to change the name in many places, and if they forgot (which happens), something would break. It wouldn't be that bad if it was only NameError: uninitialized constant Bst, but what would happen if at the same time that someone renamed some other class to Bst? Beside that, a commit renaming a class with only one line diff in that class file is much nicer :).

With advanced refactoring tools we all (hopefully) use it probably doesn't matter that much, but I don't like relying on explicit class names when I don't have to.

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to Binary Search Tree in the Ruby Track