老师，这段代码可以accept，但是only faster than 5.76% ，想不到什么优化方案了，老师有没有什么建议？

public List<List<Integer>> combinationSum(int[] candidates, int target) {
        List<List<Integer>> result = new ArrayList<>();
        if (candidates == null || candidates.length == 0) {
            return result;
        }
        Arrays.sort(candidates);
        boolean hasAnswer = candidates[0] <= target;
        if (!hasAnswer) {
            return result;
        }
        for (int i = 0; i < candidates.length; i++) {
            int selected = candidates[i];
            if (selected > target) {
                return result;
            }
            if (target == selected) {
                List<Integer> oneAnswer = new ArrayList<>();
                oneAnswer.add(target);
                result.add(oneAnswer);
                return result;
            } else {
                List<List<Integer>> lists = combinationSum(candidates, target - selected);
                for (List<Integer> list : lists) {
                    list.add(selected);
                    Collections.sort(list);
                    if (!result.contains(list)) {
                        result.add(list);
                    }
                }

            }

        }
        return result;
    }