If you want to view kata that you have solved, you can do so in your profile. There it shows you which languages you completed for that kata. It will give you the basic functionality that you are asking for.

While it would be great to have kata I've completed show up in the searchable kata list based off of which language you have selected, due to the complexity of different configurable filters, it would require some rework to support "kata i've completed" at a language level. Its not likely to change any time soon.

To help me understand more: Where would it display this and how would that help you learn more languages at the same time? Right now you can favorite (and soon collect) any kata that you wish to remember for later - just click the star icon under the title.

Your solution is highly inefficient: sorting takes n log n time, reversing n/2 times, while you can do it all in n time.

Also, changing the array in place when not required is bad practice.

you need to look closer at how you construct your string and what is expected.