def append(array, n)
  return array if n < 0
  array << n
  append(array, n-1)
end

def reverse_append(array, n)
  return array if n < 0
  reverse_append(array, n-1)
  array << n
end

> p append([], 5) #=> Should give [0,1,2,3,4,5]
[5, 4, 3, 2, 1, 0]
> p reverse_append([], 5) #=> Should give [5,4,3,2,1,0]
[0, 1, 2, 3, 4, 5]