258. Add Digits

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

Example:

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

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

Rust Solution

struct Solution;

impl Solution {
    fn add_digits(mut num: i32) -> i32 {
        while num > 9 {
            num = Self::add_digits_once(num);
        }
        num
    }
    fn add_digits_once(mut num: i32) -> i32 {
        let mut res = 0;
        while num > 0 {
            res += num % 10;
            num /= 10;
        }
        res
    }
}

#[test]
fn test() {
    assert_eq!(Solution::add_digits(38), 2);
}

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