public int[] twoSum(int[] nums, int target) {
        if (nums.length == 0) return null;
        Map<Integer, Integer> map = new HashMap<>();

        for(int i = 0; i < nums.length; i++) {
            int a = target - nums[i];
            if (map.get(a) != null) {
                return new int[]{map.get(a), i};
            }
            map.put(nums[i], i);
        }
        return null;
    }