10/05/2018

[Java] Total votes count as of given timestamp

Given a list of timestamped votes, return the top K voted people (including ties) as of a given timestamp.
Add an input parameter to specify if the ranking should be done olympic style, eg: if 2 people are in first place, then the next place is 3rd and not 2nd; in this case, for skipped places, return an empty list.

This is NOT a streaming problem, since the list of votes is given fully in input. Since it includes some sorting, it can be solved in O(N log N) time and O(N) space.

The input is given as a list of Vote objects and we use an auxiliary structure, CandidateVote for our max heap.

The idea is to use a map from candidate to total votes count to track the total votes for each candidate as of the given timestamp (do NOT consider votes AFTER the given time) by scanning the full input in O(N).
Then, we use a MAX heap (new PriorityQueue(Collections.reverseOrder());) along with our comparator to order them descending.

Now we simply need to pick ALL the candidates tied at the same place for each position up to K. This gets a bit more complicated if we do the olympic ranking, since we need to skip places if many people are tied in one. For example:

A - 2
B - 2
C - 1

with a topK = 2 would return 1st: A,B - 2nd: empty, but with a topK = 3 would return 1st A,B - 2nd: empty - 3rd: C.

Gotchas: the usual suspects, our heap might become empty at some point, so check for it to avoid NullPointerExceptions. If we are doing olympic ranking, we need to advance in position UP TO K, while still filling the skipped ones with empty lists.

This is another exercise where proper testing goes a long way towards spotting these small mistakes, also PriorityQueue comes to the rescue one again :)

You can check my implementation of getVoteRanking on my Gist alongside the previously mentioned auxiliary data structures and some test in VoteRankingJTests.

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