[leetcode] add digit

class Solution {
public:
    int DigitSum(int num){
        int sum=0;
        do{
            sum=sum+num%10;
            num=num/10;
        }while(num!=0)
        return sum;
    }
    int addDigits(int num) {
        int n=num;
        while(n>=10){
            num=DigitSum(n); 
            n=num;
        }
        return num;
    }
};