conversion to string and lookup in it can't be efficient

looks like String(bool) comparison against "true"

There is no precision for the output of 2 digits implemented

u can replace String(number).characters to numbers.description.compactMap { $0 }

cheater)))))

More random tests should be better (like, 10-20).

amazing.

It's literally in the title of this kata, and in the description:
Trianguluar Numbers

How you created formulae for this sequence of numbers

Damn! That's genious

Nice and elegant solution!
But doesn't work with swift5.
Consider to delete ".characters"

What do you mean by "optimization"?
Both .description and String() work the same way for integers - String initializer, Integer description

Edit:
Oh, I've noticed what you meant - number.description.sorted {...}? Not sure about optimization, but easier to read for sure.

This is the easiest to read. I just used a regular if statement though.

Looks good. You can optimize this by replacing String(number).characters with number.description