class Solution {
    public List<List<Integer>> combinationSum3(int k, int n) {
        List<List<Integer>> res = new ArrayList<>();
        backtracking(res, new ArrayList<Integer>(), 1, k, 0, n);
        return res;
    }

    private void backtracking(List<List<Integer>> res, List<Integer> path, int index, int k, int curr, int target) {
        if (path.size() == k) {
            if (curr == target) {
                res.add(new ArrayList<>(path));
            }

            return;
        }

        if (curr > target || path.size() > k) return;

        for (int i = index; i <= 9; i++) {
            path.add(i);
            backtracking(res, path, i + 1, k, curr + i, target);
            path.remove(path.size() - 1);
        }
    }
}