flatten.rb

def flatten src_array
  flat = []
  # It would be more readable to just call flat.concat(flatten(...)) recurisvely
  # but that creates and destroys a lot of intermediate arrays, which may be costly
  append_all(flat, src_array)
  flat
end

def append_all dest_array, src_array
  src_array.each do |value|
    if value.kind_of?(Array)
      append_all(dest_array, value)
    else
      dest_array << value
    end
  end
end

# Ordinarily I'd add some tests here, but as I don't even know if I meet the
# requirements for this job, I think I've spent enough time on this.
#
# flatten([1,2,[3,[4,5]],[6,7]])
# => [1, 2, 3, 4, 5, 6, 7]
# flatten([[1,2,[3]],4])
# => [1, 2, 3, 4]
# flatten([[1,2,[3,5,[8,9]]],4])
# => [1, 2, 3, 5, 8, 9, 4]