You are given an integer array nums and you have to return a new counts array.
The counts array has the property where counts[i] is the number of
smaller elements to the right of nums[i].
Example:
Input: [5,2,6,1]
Output: [2,1,1,0]
Explanation:
To the right of 5 there are 2 smaller elements (2 and 1).
To the right of 2 there is only 1 smaller element (1).
To the right of 6 there is 1 smaller element (1).
To the right of 1 there is 0 smaller element.