public List<List<Integer>> combinationSum2(int[] candidates, int target) {
         List<List<Integer>> res = new ArrayList<List<Integer>>();
       Arrays.sort(candidates);
        backtrack(res,new ArrayList<Integer>(),target,candidates,0);
        return res;
    }

    public void backtrack(List<List<Integer>> res, List<Integer> tempList, int remain, int[] candidates, int start){
        if(remain<0) return;
        else if(remain==0) res.add(new ArrayList<>(tempList));
        else{
            for(int i=start;i<candidates.length;i++){
                if(i>start&&candidates[i]==candidates[i-1])continue;
                tempList.add(candidates[i]);
                backtrack(res, tempList, remain-candidates[i],candidates, i+1);//i+1 means cannot reuse same element
                tempList.remove(tempList.size()-1);
            }
        }
    }