Find all valid combinations of `k` numbers that sum up to `n` such that the following conditions are true:

- Only numbers `1` through `9` are used.
- Each number is used at most once.

Return a list of all possible valid combinations. The list must not contain the same combination twice, and the combinations may be returned in any order.

Example 1:

    Input: k = 3, n = 7
    Output: [[1,2,4]]
    Explanation:
    1 + 2 + 4 = 7
    There are no other valid combinations.

Example 2:

    Input: k = 3, n = 9
    Output: [[1,2,6],[1,3,5],[2,3,4]]
    Explanation:
    1 + 2 + 6 = 9
    1 + 3 + 5 = 9
    2 + 3 + 4 = 9
    There are no other valid combinations.

Example 3:

    Input: k = 4, n = 1
    Output: []
    Explanation: There are no valid combinations.
    Using 4 different numbers in the range [1,9], the smallest sum we can get is 1+2+3+4 = 10 and since 10 > 1, there are no valid combination.

Constraints:

- `2 <= k <= 9`
- `1 <= n <= 60`