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leetcode 258. 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?


class Solution {
public:
    int addDigits(int num) {
        if(num <= 9) {
            return num;
        }
        else {
            return (num % 9 == 0)?9:(num % 9);
        }
    }
};