Find a number where it appears exactly N/2 times

Question

Here is one of my interview question. Given an array of N elements and where an element appears exactly N/2 times and the rest N/2 elements are unique

Answer 1

Contrary to answers above, there is a solution with worst case behavior as requested, O(log n) RUN TIME. The problem is not to find a solution with O(log N) comparisons worst case (which is impossible), but to do it O(log N) time.

If you can do N comparisons in parallel, the solution is a trivial divide-and-conquer. Not very practical in the real world, but it's an interview question, not a real-world problem.

Update: I think you can do it in constant time with O(N) processors