How to help new employees build a sense of belonging in a short period of time?

It usually takes about half a year for a new employee to get rid of the feeling of being an outsider, and it may take three years or even longer for one to truly understand his or her own company. But the hardest problem to solve is actually the new employee's understanding and integration into the company culture. How to make the new staff融入into the existing company at the fastest speed? How to achieve the optimal result in terms of input-output ratio for new employees?