258. Add Digits

Description

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:

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

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

class Solution {
public:
    int addDigits(int num) {
        return (num - 1) % 9 + 1;
    }
};

class Solution:
    def addDigits(self, num: int) -> int:
        return 0 if num == 0 else (num - 1) % 9 + 1

func addDigits(num int) int {
	if num == 0 {
		return 0
	}
	return (num-1)%9 + 1
}

impl Solution {
    pub fn add_digits(mut num: i32) -> i32 {
        ((num - 1) % 9) + 1
    }
}