The one-digit oracle

Problem

The tower of Ormund has no lock on its door. A stone oracle guards it instead, and the oracle only understands single digits. When a traveler speaks a number, the oracle adds up its digits; if the result has more than one digit, it adds up the digits of that result, and it keeps going until only one is left. That digit opens the door.

Write a function that takes a whole number, 0 or greater, and returns that final digit as a whole number. With 4783: 4 + 7 + 8 + 3 = 22, which has two digits; 2 + 2 = 4, which has one. You return 4.

A number that already has a single digit stays as it is: 7 returns 7, and 0 returns 0.

Examples

• A single round

  16 → 7

• Two rounds

  942 → 6

• Two rounds, from two digits

  38 → 2

• Already one digit

  7 → 7

Besides these, the challenge has hidden tests that are revealed when you submit your solution.

You start with this

def digital_root(n):
    pass

function digitalRoot(n) {
}