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 {
    public List<List<Integer>> combinationSum3(int k, int n) {
        List<List<Integer>> res = new ArrayList<List<Integer>>();
       
        helper(k, n, 1, 0, new ArrayList<Integer>(), res);
        return res;
    }
   
    private void helper(int k, int n, int cur, int curSum, List<Integer> curList, List<List<Integer>> res) {
        if (curSum == n && curList.size() == k) {
            List<Integer> newList = new ArrayList<Integer>(curList);
            res.add(newList);
        } else if (curSum < n && curList.size() < k) {
            for (int i = cur; i <= 9; i++) {
                curList.add(i);
                helper(k, n, i + 1, curSum + i, curList, res);
                curList.remove(curList.size() - 1);
            }
        }
    }
}