SICP Exercise 1.11.rb

def recursive(n)
  if n < 3
    n
  else
    recursive(n - 1) +
      2 * recursive(n - 2) +
      3 * recursive(n - 3)
  end
end

def iterative(n)
  return n if n < 3

  c = 0
  b = 1
  a = 2

  3.upto(n).each do
    x = a + 2*b + 3*c
    c = b
    b = a
    a = x
  end

  a
end