Flat Array

flat.rb

def flat(nested)
  return nil if nested.nil?
  res = []
  dfs(nested, res)
  return res
end

# use DFS to search all integers
def dfs(item, res)
  if !item.is_a?(Array)
    res << item
  else
    item.each {|i| dfs(i, res)}
  end
end

flat_test.rb

require './flat'
require 'minitest'
require 'minitest/autorun'

class FlatTest < Minitest::Test
  def test_nil
    assert_equal flat(nil), nil
  end

  def test_empty
    assert_equal flat([]), []
  end

  def test_nested_empty
    array = [[], [], [[]]]
    assert_equal flat(array), []
  end

  def test_not_nested
    array = [1, 2, 3, 4]
    res = [1, 2, 3, 4]
    assert_equal array, res
  end

  def test_nested
    array = [[1, 2, [3]], 4]
    res = [1, 2, 3, 4]
    assert_equal flat(array), res
  end

  def test_nested_include_nil
    array = [[1, 2, [nil]], 4]
    res = [1, 2, nil, 4]
    assert_equal flat(array), res
  end
end

Gemfile

source 'https://rubygems.org'

group :test do
  gem 'minitest'
end