The OP's problem is counting unique elements. Sorting is implicit in that problem, whether by hashing (sort into buckets according to arbitrary hash function), or straight up sort.

Generic sort is O(n log n), but there is also Counting Sort, Radix Sort, and other forms of distribution sorting that are of high utility in real-world applications. It is important to understand that hashing is not algorithmically superior to sorting, indeed it is a specific form of sorting in disguise.

For another example of a similar problem, and how sort can save the day, see Perl Hashes in C?.

In reply to Re^3: Out of Memory when generating large matrix by Anonymous Monk
in thread Out of Memory when generating large matrix by cathyyihao

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