problem

Given an integer num, repeatedly add all its digits until the result has only one digit, and return it.

 

Example 1:

Input: num = 38
Output: 2
Explanation: The process is
38 --> 3 + 8 --> 11
11 --> 1 + 1 --> 2 
Since 2 has only one digit, return it.

Example 2:

Input: num = 0
Output: 0

 

Constraints:

  • 0 <= num <= 231 - 1

 

Follow up: Could you do it without any loop/recursion in O(1) runtime?

submission

// check https://en.wikipedia.org/wiki/Digital_root#Congruence_formula
impl Solution {
    pub fn add_digits(num: i32) -> i32 {
        (num - 1) % 9 + 1
    }
}