swerc2014 Book Club(二分图匹配)

Porto’s book club is buzzing with excitement for the annual book exchange event! Every year, members bring their favorite book and try to find another book they like that is owned by someone willing to trade with them. I have been to this book exchange before, and I definitely do not want to miss it this year, but I feel that the trading should be improved. In the past, pairs of members interested in each other’s books would simply trade: imagine that person A brought a book that person B liked and vice-versa, then A and B would exchange their books. I then realized that many members were left with the same book they walked-in with... If instead of looking for pairs I looked for triplets, I could find more valid exchanges! Imagine that member A only likes member B’s book, wh