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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.
O(N^2) is the lower bound for this problem for both time and space. Slicing aside, even just creating a string is an O(len) operation, and the output needs to have N strings of length N.
Seems like this is still an issue