Combination Sum

//target=2, candidates=[1, 2]
public class Solution {
    private LinkedList<LinkedList<Integer>> results=new LinkedList<LinkedList<Integer>>();
    public LinkedList<LinkedList<Integer>> combinationSum(int[] candidates, int target) {
    Arrays.sort(candidates);    
    LinkedList<Integer> solution=new LinkedList<Integer>();    
    helper(candidates, 0, target, solution);
    return results;
    }
    
    private void helper(int[] candidates, int k, int target, LinkedList<Integer> solution) {
        if (target<0) return;
        if (target==0) { 
            LinkedList<Integer> res=new LinkedList<Integer>(solution);
            results.add(res);
            return;
        }
        
     
     for (int i=k; i<candidates.length; i++) {
         solution.add(candidates[i]);
         helper(candidates, i, target-candidates[i], solution);
         solution.remove(solution.size()-1);
     }
     
    }
}