[leetCode] Combination Sum III

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]]

public class Solution {
List<List<Integer>> res = new ArrayList<List<Integer>>();

public List<List<Integer>> combinationSum3(int k, int n) {
if (k > 9 || n > 45) return res;
List <Integer> tmp = new ArrayList<Integer>();
for (int i = 1; i <= 9 && i <= n; i++) {
helper(tmp, i, 0, k, n);
}

return res;
}

private void helper( List<Integer> list, int num, int sum, int k, int n) {
if (sum + num == n && list.size() == k - 1) {
list.add(num);
List<Integer> tmp = new ArrayList<Integer>(list);
res.add(tmp);
list.remove(list.size()-1);
}
else {
if (list.size() == k - 1 || sum + num > n) return;
list.add(num);
sum += num;

for (int i = num + 1; i <= 9; i++) {
helper(list, i, sum, k, n);
}
list.remove(list.size()-1);
}
}
}