Given an integer array nums, return an array counts where counts[i] is the number of smaller elements to the right of nums[i].
Example: nums returns . - Right of 5: 2 and 1 are smaller → 2 - Right of 2: 1 is smaller → 1 - Right of 6: 1 is smaller → 1 - Right of 1: nothing → 0 This is "count inversions" per element, solvable with segment tree or BIT. Constraints: up to elements, values up to .