You use modified merge sort. While merging, count pairs where left element > 2 * right element.
The trick is: during merge, both halves are sorted. You use two pointers: for each left element, count right elements where condition holds.
Example: [1,3,2,3,1]. When merging [1,3] and [2,3,1], you count pairs: 3 > 2*1 (yes).
Modified merge sort. For : time, about operations. space.