Find all possible combinations ofknumbers that add up to a numbern, given that only numbers from 1 to 9 can be used and each combination should be a unique set of numbers.
Example 1:
Input:k= 3,n= 7 Output: [[1,2,4]]
Example 2:
Input:k= 3,n= 9 Output: [[1,2,6], [1,3,5], [2,3,4]]
