public List<Integer> countSmaller(int[] nums) {

        List<Integer> res = new ArrayList<>();

        int n = nums.length;

        int[] nums_copy = Arrays.copyOf(nums, n);

        Map<Integer, Integer> map = new HashMap<>();

        Arrays.sort(nums_copy);

        int rank = 0;

        for (int i = 0; i < n; i++) {

            if (i == 0 || nums_copy[i] != nums_copy[i - 1]) {

                map.put(nums_copy[i], ++rank);

            }

        }

        int[] bit = new int[map.size() + 1];

        for (int i = n - 1; i >= 0; i--) {

            update(bit, map.get(nums[i]));

            res.add(query(bit, map.get(nums[i]) - 1));

        }

        Collections.reverse(res);

        return res;

    }

    private void update(int[] bit, int idx) {

        for (int i = idx; i < bit.length; i += i & -i) {

            bit[i] += 1;

        }

    }

    private int query(int[] bit, int idx) {

        int res = 0;

        for (int i = idx; i > 0; i -= i & -i) {

            res += bit[i];

        }

        return res;

    }