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Editorial Team
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Asked: 2026-06-05T12:36:14+00:00

I’m looking for a data structure to hold an unordered collection of unique elements,

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I’m looking for a data structure to hold an unordered collection of unique elements, which would support the following operations

  1. insertions/deletions of elements anywhere in the collection
  2. querying if an element is present
  3. access to a random element

Naively, 1 and 2 suggest using an associative container, e.g. unordered_set, but then 3 is linear in number of elements. Using a random access container, e.g. vector, makes 3 easy, 1 can be done in O(1), but then 2 is again O(N).

The question is if there’s a known way around this linear complexity?

EDIT: By a random element in 3 I mean: given an arbitrary ordering of N elements, retrieve an element number j where j is between 0 and N-1. For an std::vector it’s just subscripting, for an std::list or an std::set it’s incrementing the list/set iterator j times starting from begin() etc.

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1 Answer

  1. Editorial Team

    I have been looking for such a data structure for a long time.

    Recently, I found quite promising library which has all the functionality that you are looking for.

    See the cntree::set with random access in O(log n).

    here is the link.
    http://dl.dropbox.com/u/8437476/works/countertree/index.html

    Although it seems to be under development, I see it is quite usable.

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