class Solution {
    public List<List<Integer>> subsets(int[] nums) {
        List<List<Integer>> res = new ArrayList<>();
        backtracking(res, new ArrayList<Integer>(), 0, nums);
        return res;
    }

    // Each level it walks from start to end,
    // and recursively process the remainder
    // Becuase all numbers are unique, so each level number
    // cannot be the same, which ensures all results are unique
    private void backtracking(List<List<Integer>> res, List<Integer> path, int start, int[] nums) {
        res.add(new ArrayList<>(path));

        for (int i = start; i < nums.length; i++) {
            // Make a new move
            path.add(nums[i]);

            // Advance to next level
            backtracking(res, path, i + 1, nums);

            // Undo move after backtracking from the level below
            path.remove(path.size() - 1);
        }
    }
}