class Solution {
private:
vector<vector<int>> res;
vector<int> path;
void backtracking(int k, int n, int sum, int startIndex) {
if (sum > n) return;
if (path.size() == k) {
if (sum == n) res.push_back(path);
return;
}
for (int i = startIndex; i <= 9 - (k - path.size()) + 1; i++) {
path.push_back(i);
backtracking(k, n, sum + i, i + 1);
path.pop_back();
}
}
public:
vector<vector<int>> combinationSum3(int k, int n) {
backtracking(k, n, 0, 1);
return res;
}
};