注意点：

1. HashMap<Integer, Integer>
2. return new int[]{};
3. 3 2 4 target:6
4. return null;

public int[] twoSum(int[] nums, int target) {

        HashMap<Integer, Integer> m = new HashMap<>();

        for(int i = 0; i < nums.length; ++i){

            if(m.containsKey(target - nums[i]))

                return new int[]{m.get(target - nums[i]), i};

            m.put(nums[i], i);

        }

        return null;

    }