for the C# solutions, I noticed no one used a dictionary...is there a reason?

Approved

Fixed in your fork

isn't it easy to read and understand to write "function". I know its easy to write arrow function, but for me the classic way is much more simpler to understand and read.

whole numbers, integers, and decimals, floats, are different kinds of numerical data. There is no need to worry about floats because that isnt the data being tested.

Doesn't this return " " instead of "" for an empty string? I thought I tried this code and it errored ...

Edit: I just tried it out and it didn't error. Never Mind. I guess it doesn't join on an empty string ("") because there is nothing to join.

No random tests??

well it took me quite long to do it and while checking others solution,it looks like how easy it was but wasn't for me and now it is.

python new test framework + random tests are required. updated in this fork

Python new test framework should be used.

More details here and here

genius! I was stuck on this one for hours

I think the callback makes the code is longer

#THIEVES118

this function, although returns a string output, there is still spaces at the begining and of the string.

yeah should be defensive