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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.
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Stopping after sqrt won't work here, you have to check till number//2+1.
What you said is correct, if the number has a large factor, it will be time consuming.Please refer to my solution.
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Using a dict cache is a very clever practice when you need to double-count. You can stop divisors calculation after sqrt(number).
Maybe you should judge by length~
The arrarys can improve readability, the only problem is that O(n) has a bigger constant.
This method does not need to search the index.
Your idea is very good, but code is not clean.