```
class Solution {
vector<vector<int>> result;
// the method is to get the combination
// r is the start number to try which must be less than 10
// v stores the numbers that have been tried
void getCom(int k, int n, int r, vector<int>& v) {
if (k == 0) {
if (n == 0) result.push_back(v);
return;
}
for (int i = r; i <= 9; ++i) {
v.push_back(i);
getCom(k - 1, n - i, i + 1, v);
v.pop_back();
}
}
public:
vector<vector<int>> combinationSum3(int k, int n) {
vector<int> v;
getCom(k, n, 1, v);
return result;
}
};
```