I believe it’s QuickSelect using median of medians as a pivot strategy; not sure if it’s O(1) space, though (standard QuickSelect is in-place - not sure how the pivoting strategy affects this).

This is actually vague question as there is a technique called quick select which works like quick short but it has worst case time complexity O(n²) so using that is risky and also in best case also amortized time complexity is O(n). I think the interviewer is just checking if you know the technique. I would recommend using heap or sorting for this problem if came in some contest.