My first solution was to use a sequential search - O(1) space and O(n^2) time. My second solution was to use a bitmap index, assuming we could allocate enough words to represent the entire range of integers as sequential bits - O(n) worst case time. Other possible solutions included doing an on disk bucket sort using in place quicksort (or mergesort) as the sub-sort.
No difficulty in their questions during interviews save for the online test by a 3rd party. Probably one of the easier jobs I have applied for in terms of question difficulty, They offset this of course with their long interview process that has the potential to lead nowhere.
They used up 5 hours of my time, 4 of which I had to make up at my current job. They waste your time and are not apologetic for it. They string you along without confirmation that a role is actually available. They keep you in a talent pool/hold on to your data and believe this is somehow a good consolation prize for not getting an offer.