---
description: >-
  Author: @wingkwong |
  https://leetcode.com/problems/calculate-digit-sum-of-a-string/
---

# 2243 - Calculate Digit Sum of a String (Easy)

## Problem Link

https://leetcode.com/problems/calculate-digit-sum-of-a-string/

## Problem Statement

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.

## Approach 1: Simulation

Just following the instructions.

<SolutionAuthor name="@wingkwong"/>

```cpp
class Solution {
public:
    string digitSum(string s, int k) {
        while (s.size() > k) {
            string ns = "";
            for (int i = 0; i < s.size(); i += k) {
                // Divide s into a group of size k
                string t = s.substr(i, k);
                // Calulate the digit sum in the current group
                int sum = 0;
                for (int j = 0; j < min((int) t.size(), k); j++) {
                    sum += t[j] - '0';
                }
                // merge back together
                ns += to_string(sum);
            }
            // replace s and do the next round
            s = ns;
        }
        return s;
    }
};
```
