flatten.rb

def flatten(arr, output = [])
  raise "Input must be array. Usage: flatten(arr)" unless arr.class == Array
  arr.each do |el|
    if el.class == Array
      flatten(el, output)
    else
      output << el
    end
  end
  output
end

# Examples
# >   flatten([])
#  => []
# > flatten([1])
#  => [1]
# > flatten([1, 2])
#  => [1, 2]
# > flatten([1, [2]])
#  => [1, 2]
# > flatten([[1, 2, 3], [4, 5, 6, [7, 8]], 9, 10])
#  => [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
# > flatten([[1, 2, 3], [4, 5, 6, [7, 8]], 9, 10, [[[[[11]]]]]])
#  => [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]