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Are you trying to find a unique set of strings, or a unique set of characters within a single string?

Currently it looks like you're trying to do the first. Also, you're returning the original...

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Hint: use the iterator of the the TreeMap and a while loop. You can keep the same collection, or place all items left into a new collection (e.g. a List or a Set).

hmm... I don't think a HashSet is the best way to do this. Rather, I would use a Map (preferably a TreeMap). Use characters as the key, and an integer counter of how many times that character...