Given an integer array nums, return an integer array counts where counts[i] is the number of smaller elements to the right of nums[i].
Input: nums = [5,2,6,1]
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.