Find all possible combinations of k numbers that add up to a number n, given that only numbers from 1 to 9 can be used and each combination should be a unique set of numbers.
Input: k = 3, n = 7
Input: k = 3, n = 9
[[1,2,6], [1,3,5], [2,3,4]]
Special thanks to @mithmatt for adding this problem and creating all test cases.