Find all valid combinations of k numbers that sum up to n such that only numbers 1 through 9 are used and each number is used at most once.
Return a list of all possible valid combinations.
Example 1:
Input: k = 3, n = 7 Output: [[1,2,4]] Explanation: 1 + 2 + 4 = 7
Example 2:
Input: k = 3, n = 9 Output: [[1,2,6],[1,3,5],[2,3,4]]
Constraints:
2 <= k <= 91 <= n <= 60