2243. Calculate Digit Sum of a String

You are given a string s consisting of digits and an integer k.

A round can be completed if the length of s is greater than k. In one round, do the following:

1. Divide s into consecutive groups of size k such that the first k characters are in the first group, the next k characters are in the second group, and so on. Note that the size of the last group can be smaller than k.
2. Replace each group of s with a string representing the sum of all its digits. For example, “346” is replaced with “13” because 3 + 4 + 6 = 13.
3. Merge consecutive groups together to form a new string. If the length of the string is greater than k, repeat from step 1.

Return s after all rounds have been completed.

class Solution {
    int parse(string& s, int &p, int k) {
        int ed = p + k, res = 0;
        while(p < s.length() and p < ed) {
            res = res + (s[p++] & 0b1111);
        }
        return res;
    }
public:
    string digitSum(string s, int k) {
        while(s.length() > k) {
            int i = 0;
            string ns = "";
            while(i < s.length())
                ns += to_string(parse(s, i, k));
            s = ns;
        }
        return s;
    }
};