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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.

def digital_root(n)
    arr = n.to_s.chars.map(&:to_i)
    sum = arr.inject(0) { |memo, n| memo + n }
    if sum.to_s.size == 1
        return sum
    else
        digital_root(sum)
    end
end

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