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.
Note:
All numbers will be positive integers.
The solution set must not contain duplicate combinations.
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]]