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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.
Some googling suggests that when you concatenate or take a substring of a Java string, it makes a copy. This takes O(n) time, like iterating over all the characters. Maybe you could avoid this by casting to a char array and then mutating it, but that is not idiomatic to Clojure.
O(n^2) because the for comprehension generates a cartesian product of the numbers vector with itself (or more specifically in this case, the indices of the numbers vector)