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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.
Agreed. This is how I lost 4 hours of my life. Probably more...
Read the description again. It says take letters from those strings and return a new one, see the examples. Also, the title of the kata is Two to One, not One of Two. Not a kata issue.
The exercise makes sense after reading this explanation.
The exercise doesn't mention that we should combine the letter from s1 and s2 into a single block of letter. It seems to say either choose s1 or choose s2 depending on which has the longest result.