Leetcode 258

Simple reccurssion, you have a base case where you have just 1 digit, simply return it, you have recussive case, you add all the digit and return addDigits adds all these digit recurssively

class Solution {
    public int addDigits(int num) {
        // base case
        if(num < 10) return num;
        int res = 0;
        while(num != 0){
            res += num % 10;
            num /= 10;
        }
        return addDigits(res);
    }
}