leetcode Add digits

Given a non-negative integer num, repeatedly add all its digits until the result has only one digit.

For example:

Given num = 38, the process is like: 3 + 8 = 11, 1 + 1 = 2. Since 2 has only one digit, return it.

思路:

digit root 问题：

公式就是1+(n-1)%9

https://en.wikipedia.org/wiki/Digital_root#Congruence_formula

int addDigits(int num) {    return (1+(num-1)%9);}