Description (leetcode)

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 <= 2^31 - 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
    }
}