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.

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]]

void combinationSum3(vector<vector<int>>&ret, vector<int>&comb, int k, int n,int &sum, int begin)
     {
         if (sum > n)return;
         if (comb.size() == k && sum != n)return;         
         else if (comb.size() == k && sum == n) 
         {
             ret.push_back(comb);
             return;
         }
         for (int i = begin; i <= 9;i++)
         {
             comb.push_back(i);
             sum += i;
             combinationSum3(ret, comb, k, n, sum, i + 1);
             sum -= i;
             comb.pop_back();
         }
     }
     vector<vector<int>> combinationSum3(int k, int n)
     {
         vector<vector<int>> ret;
         vector<int> comb;
         int sum = 0;
         combinationSum3(ret, comb, k, n, sum, 1);
         return ret;
     }