Hello guys, the combination sum series problems are traditional backtracking problems, just like the N-queens,they are worth to try and you'll figure out the concept of backtracking.

Fight for interviews!

```
public List<List<Integer>> combinationSum3(int k, int n) {
List<List<Integer>> list = new ArrayList<>();
backtrack(list, new ArrayList<Integer>(), k, n, 1);
return list;
}
private static void backtrack(List<List<Integer>> list, ArrayList<Integer> temp, int k, int sum, int start) {
if (sum < 0) {
return;
} else if (temp.size() == k && sum == 0) {
list.add(new ArrayList<Integer>(temp));
} else {
for (int i = start; i <= 9; i++) {
temp.add(i);
backtrack(list, temp, k, sum - i, i + 1);
temp.remove(temp.size() - 1);
}
}
}
```