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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.
You are using == to compare the Strings, but by doing so you are comparing the memory address and not the String values. To fix this you can compare using the equals method or simply do dna.charAt(i) == 'T' (with single quotes), for example, and your code will work.