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to Binary Search Tree in the Ruby Track

Published at Jul 13 2018 · 0 comments
Instructions
Test suite
Solution

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

   4
 /   \
2     6
 \
  3

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 Ruby resources 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
end
# frozen_string_literal: true

class Bst
  attr_reader :data, :left, :right

  def initialize(data)
    @data = data
  end

  def insert(value)
    if data >= value
      branch_insert("left", value)
    else
      branch_insert("right", value)
    end
  end

  def each(&block)
    return enum_for(:each) unless block_given?

    left&.each(&block)
    yield(data)
    right&.each(&block)
  end

  private

  attr_writer :left, :right

  def branch_insert(branch, value)
    if (node = public_send(branch))
      node.insert(value)
    else
      send("#{branch}=", self.class.new(value))
    end
  end
end

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