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.

Note:

• All numbers will be positive integers.
• The solution set must not contain duplicate combinations.

class Solution {
    public List<List<Integer>> combinationSum3(int k, int n) {
        List<List<Integer>> res = new ArrayList<>();
        if(n <= 0 || k <= 0)
            return res;

        int[] nums = new int[9];
        for(int i = 0; i < nums.length; i++)
            nums[i] = i + 1;

        List<Integer> line = new ArrayList<>();
        helper(res, line, nums, 0, k, n);

        return res;
    }

    public void helper(List<List<Integer>> res, List<Integer> line, int[] nums, int start, int k, int n) {
        if(line.size() == k) {
            if(n == 0) {
                res.add(new ArrayList<>(line));
            }

            return;
        }

        if(start == nums.length)
            return;

        for(int i = start; i < nums.length; i++) {
            line.add(nums[i]);
            helper(res, line, nums, i + 1, k, n - nums[i]);
            line.remove(line.size() - 1);
        }
    }
}