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.

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]]

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public class Solution {
    public List<List<Integer>> combinationSum3(int k, int n) {
        List<List<Integer>> result = new ArrayList<>();
        sum(result, new ArrayList<Integer>(), n, k, 1);
        return result;
    }
    
    public void sum(List<List<Integer>> result, ArrayList<Integer> cur, int target, int k, int num) {
        if (target == 0 && cur.size() == k) {
            result.add(new ArrayList<Integer>(cur));
        } else if (target > 0 && cur.size() < k) {
            for (int start = num; start < 10; start++) {
                cur.add(start);
                sum(result, cur, target - start, k, start + 1);
                cur.remove(cur.size() - 1);
            }
        }
    }
}