LintCode-569.各位相加

class Solution {
public:
    /**
     * @param num a non-negative integer
     * @return one digit
     */
    int addDigits(int num) {
        // Write your code here
        int sum = num;
        while(sum >= 10){
            sum = 0;
            while(num > 0){
                sum += num % 10;
                num = num / 10;
            }
            num = sum;
        }
        return sum;
    }
};