#$6.($Not in the book$)--$Rumor spreading $-$ There are $n$ people, each in possession of a different rumor. They want to share all the rumors with each other by sending electronic messages. Assume that a sender includes all the rumors he or she knows at the time the message is sent and that a message may only have one addressee. Design a greedy algorithm that always yields the minimum number of messages they need to send to guarantee that everyone of them gets all the rumors.