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.
- All numbers will be positive integers.
- The solution set must not contain duplicate combinations.
Input: (k = 3), (n = 7)
Output: [[1, 2, 4]]Input: (k = 3), (n = 9)
Output: [[1, 2, 6], [1, 3, 5], [2, 3, 4]]