C# One-Liner, Recursion, LINQ
\npublic class Solution \n{\n    public string DigitSum(string s, int k) \n        => s.Length <= k ? s : DigitSum(String.Concat(s.Chunk(k).Select(g => g.Sum(c 
```\npublic class Solution \n{\n    public string DigitSum(string s, int k) \n        => s.Length <= k ? s : DigitSum(String.Concat(s.Chunk(k).Select(g => g.Sum(c => c - \'0\'))), k);\n}\n```
['Recursion']