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§Tuning Trouble
One solution to this problem is to use the windows method to slide over groups of the desired
size, then construct a HashSet from the characters. If the HashSet is the same size
as the window then we know that all characters are unique, as sets contain no duplicate elements.
We’ll use a faster approach that minimizes the work needed. Instead of creating a set for each window, we’ll maintain the last position seen of each character. As we advance character by character we lookup the previous position. If this is within the packet size, then we advance the start of the packet to exclude that character. Once the packet has reached the desired size then we return the current index.