Calculate Digit Sum of a String.java

// Runtime: 6 ms (Top 32.40%) | Memory: 40.8 MB (Top 89.31%)
class Solution {
    public String digitSum(String s, int k) {
        while(s.length() > k) s = gen(s,k);
        return s;
    }
    public String gen(String s,int k){
        String res = "";
        for(int i=0;i < s.length();){
            int count = 0,num=0;
            while(i < s.length() && count++ < k) num += Character.getNumericValue(s.charAt(i++));
            res+=num;
        }
        return res;
    }
}