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Collections are a way for you to organize kata so that you can create your own training routines. Every collection you create is public and automatically sharable with other warriors. After you have added a few kata to a collection you and others can train on the kata contained within the collection.
Get started now by creating a new collection.
Yep, this is O(n²) for the reason you explained.
And I disagree with RazNaot, just because you can solve a problem inefficiently doesn't mean you should. The goal of this website is to improve at programming.
The custom in this site for the easier katas, is not to be restricted by performance and memory constraints. At most katas leveled kyu 8 to kyu 6, almost all algorithms will work. They are meant to be easy training, with fewer and smaller test cases - so just do something that works and you'll pass. The harder katas - around kyu 5,4 and up may require "performance" - i.e. slower algos may fail etc. Browse "Kata" on the top left menu to get to harder questions...