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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?

1. public int AddDigits(int num) {
2. char[] nums = num.ToString().ToCharArray();
3. if (num / 10 >= 1) {
4. int sum = 0;
5. foreach (char c in nums) {
6. sum += (Convert.ToInt32(c) - 48);
7. }
8. return AddDigits(sum);
9. } else {
10. return num;
11. }
12. }

来自为知笔记(Wiz)