The intended solution uses Fenwick Tree.

Let’s describe the solution for counting the number of elements smaller than nums[i] in the range [0, i - 1], and counting in the range [i + 1, nums.length - 1] can be done similarly by simply reversing the array.

Iterate from left to right and maintain the Fenwick Tree. Save the value of the query(nums[i]) and update nums[i] + 1 by 1.