public class Solution {
    public List<List<Integer>> combinationSum(int[] candidates, int target) {
        List<List<Integer>> res=new ArrayList<>();
        if(candidates.length==0) return res;
        helper(candidates,new ArrayList<>(),target,0,res);
        return res;
    }
    public void helper(int[] candidates,List<Integer> list, int target,int start,List<List<Integer>> res){
        if(target<0) return;
        if(target==0){
            res.add(new ArrayList<>(list));
            return;
        }
        for(int i=start;i<candidates.length;i++){
            list.add(candidates[i]);
            helper(candidates,list,target-candidates[i],i,res);
            list.remove(list.size()-1);
        }
    }
}