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.

Ensure that numbers within the set are sorted in ascending order.

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]]

 class Solution {

 public:

     void assist(vector<vector<int> >& res, vector<int>& cur, int target, int k, int st)

     {

         if (k == )

         {

             if (target == )

                 res.push_back(cur);

             return;

         }

         for (int i = st; i <= ; i++)

         {

             cur.push_back(i);

             assist(res, cur, target - i, k - , i + );

             cur.pop_back();

         }

     }

     vector<vector<int>> combinationSum3(int k, int n) {

         vector<vector<int> > res;

         vector<int> cur;

         assist(res, cur, n, k, );

         return res;

     }

 };