flatten.rb

def flatten(array)
  new_arr = []
  array.each do |item|
    if item.is_a?(Array)
      flatten(item).each do |inner|
        new_arr << inner
      end
    else
      new_arr << item
    end
  end

  return new_arr
end

puts flatten([[1,2,[3]],4]).inspect
#=> [1, 2, 3, 4]

puts flatten([[[[1,2],[3,4],[5,6],[7]]]]).inspect
#=> [1, 2, 3, 4, 5, 6, 7]

puts flatten([]).inspect
#=> []

puts flatten([[[[[[[[[[[1]]]]]]]]]]]).inspect
#=> [1]

puts flatten([3,4,5,6]).inspect
#=> [3, 4, 5, 6]