I got this for my interview:
Numbers are said to be "reverse ordered" if N[i] > N[j] for i < j. For example, in a list: 3 4 1 6 7 3, the reverse ordered items are (3,1) (4,1) (4,3) (6,3) (7,3).
How to get the number of pairs of reverse ordered items in O(nlogn) time.