258.整形每个组成数字相加 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.

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

public int AddDigits(int num) {
	char[] nums = num.ToString().ToCharArray();
	if (num / 10 >= 1) {
		int sum = 0;
		foreach (char c in nums) {
			sum += (Convert.ToInt32(c) - 48);
		}
		return AddDigits(sum);
	} else {
		return num;
	}
}

来自为知笔记(Wiz)