sum of digits / digital root

In this kata, you must create a digital root function.

A digital root is the recursive sum of all the digits in a number. Given n, take the sum of the digits of n. If that value has two digits, continue reducing in this way until a single-digit number is produced. This is only applicable to the natural numbers.

Here’s how it works (Ruby example given):

digital_root(16)
  => 1 + 6
  => 7

digital_root(942)
  => 9 + 4 + 2
  => 15 ...
  => 1 + 5
  => 6

digital_root(132189)
  => 1 + 3 + 2 + 1 + 8 + 9
  => 24 ...
  => 2 + 4
  => 6

digital_root(493193)
  => 4 + 9 + 3 + 1 + 9 + 3
  => 29 ...
  => 2 + 9
  => 11 ...
  => 1 + 1
  => 2

Answers:

1
2
3
4
5
6
7
8
9
10

def (n)
  
  n = n.to_s.chars.map(&:to_i).reduce(:+)
  n > 9 ? digital_root(n) : n
end

def (n)
  
  n < 10 ? n : digital_root(n / 10 + n % 10)
end