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.

 1 class Solution(object):
 2     def combinationSum3(self, k, n):
 3         """
 4         :type k: int
 5         :type n: int
 6         :rtype: List[List[int]]
 7         """
 8         nums = list(range(1,10))
 9         res = []
10         self.helper(nums, k, n, res, [])
11         return res
12         
13     def helper(self, nums, k, target, res, line):
14         if target == 0 and len(line) == k:
15             res.append([x for x in line])
16         
17         for i, x in enumerate(nums):
18             if x <= target:
19                 line.append(x)
20                 self.helper(nums[i+1:], k, target -x, res, line)
21                 line.pop()