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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.
This is not a performatic solution, since you will have to go through all sentence's element each time you get a new letter. It is a costly operation.
The best thing would be to transfrom 'sentence' into a hashmap, so that you could look for the letters presence in a o(1) way.