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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.
It might be a nice modification to this question to ask for the value on the nth day, then have a test that runs out of memory if you keep all of both lists in memory.
For identical strings in
compare_weight, you can check equality up front and save yourself the summing / iterating. I like this solution best otherwise!