Рекурсия

def fib(n)
  return 1 if n == 1 || n == 2
  return fib(n-1) + fib(n-2)
end

def fakt(n)
  return 1 if n == 1
  return fakt(n-1)*n
end

def sum(arr, i)
  return (i == 0) ? arr[0] : sum(arr, i--) + arr[i]
end