Ruby Implementation for `Array#flatten`, that flattens a map.

flatten.rb

# rspec flatten.rb

def flatten(array)
  return array if array == []

  head, *tail = array
  current = head.class == Array ? flatten(head) : [head]
  current + flatten(tail)
end

RSpec.describe 'flatten' do
  it 'returns [] for empty list' do
    expect(flatten([])).to eq([])
  end

  it 'returns the same list if alreay flat' do
    expect(flatten([1, 3, 5])).to eq([1, 3, 5])
  end

  it 'returns the flat list if the list is not flat' do
    expect(flatten([1, [2, 3], [3, [4, 5]], [6, [7, [8]]], 9])).to eq([1, 2, 3, 3, 4, 5, 6, 7, 8, 9])
  end
end