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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.
great!
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My solution was almost identical. If you place "seen.add(char)" inside an else then speed will be slighly improved.
If you have a one million characters text then after a few iterations 99.99% of the characters will be already in 'seen' set so that line of code could be skipped most of the time.