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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.
Yes, it's DP (left or right), and cache can speed it up.
i had to use memoization to solve it.. although looking at some of the top answers didn't look like they did