2243\. Calculate Digit Sum of a String

Easy

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_.

**Example 1:**

**Input:** s = "11111222223", k = 3

**Output:** "135"

**Explanation:**

- For the first round, we divide s into groups of size 3: "111", "112", "222", and "23".

Then we calculate the digit sum of each group: 1 + 1 + 1 = 3, 1 + 1 + 2 = 4, 2 + 2 + 2 = 6, and 2 + 3 = 5.

So, s becomes "3" + "4" + "6" + "5" = "3465" after the first round.

- For the second round, we divide s into "346" and "5".

Then we calculate the digit sum of each group: 3 + 4 + 6 = 13, 5 = 5.

So, s becomes "13" + "5" = "135" after second round.

Now, s.length <= k, so we return "135" as the answer. 

**Example 2:**

**Input:** s = "00000000", k = 3

**Output:** "000"

**Explanation:**

We divide s into "000", "000", and "00".

Then we calculate the digit sum of each group: 0 + 0 + 0 = 0, 0 + 0 + 0 = 0, and 0 + 0 = 0.

s becomes "0" + "0" + "0" = "000", whose length is equal to k, so we return "000". 

**Constraints:**

*   `1 <= s.length <= 100`
*   `2 <= k <= 100`
*   `s` consists of digits only.