Digital Root

Problem

Return the digital root of a non-negative integer n: repeatedly sum its digits until a single digit remains.

Use the closed form 1 + (n - 1) mod 9 for n > 0, and return 0 when n is 0. The modulo operation must not be replaced by digit loops if you want the constant-time version.

Hint

A positive number is congruent to its digit sum modulo 9.

Test cases

case 1

in: [0]

expected: 0

case 2

in: [9]

expected: 9

case 3

in: [38]

expected: 2

case 4

in: [12345]

expected: 6

case 5

in: [9999]

expected: 9

Starter codePython

def digital_root(n):
    # Your code here
    pass

5 test cases · run your code against them in the editor