Leetcode 216. combined sum III

Find the sum of all the sums n Of k Combination of numbers , And the following conditions are met ：

Use only numbers 1 To 9
Every number Use it once at most
return A list of all possible valid combinations . The list cannot contain the same combination twice , Combinations can be returned in any order .

Example 1:

 Input : k = 3, n = 7
 Output : [[1,2,4]]
 explain :
1 + 2