2025-08-18 11:43:36 +0000 UTC

class Solution:
    def digitSum(self, s: str, k: int) -> str:
        res, cur, group = list(map(int, s)), [], []
        i = 0
        def add_digits(num: int) -> None:
            if num == 0:
                cur.append(0)
                return
            while num > 0:
                group.append(num % 10)
                num //= 10
            group.reverse()
            cur.extend(group)
            group.clear()
        while len(res) > k:
            while i < len(res):
                add_digits(sum(res[i:i + k]))
                i += k
            res, cur = cur, res
            cur.clear()
            i = 0
        return "".join(map(str, res))