A company named RT&T has a network of n switching stations connected by m high-speed communications links. Each customer’s phone is directly connected to one station in his or her area. The engineers of RT&T have developed a prototype video-phone system that allows two customers to see each other during a phone call. However, to produce acceptable image quality, the number of links used to transmit video signals between the two parties cannot exceed four. Suppose that the RT&T network is represented by a graph. Describe an efficient algorithm that computes, for each station, the set of stations it can reach using no more than four links.