（1）根据选课表建立对应的图 （2）求出至少一种拓扑有序序列 （3）如果能实现求出所有拓扑有序序列，则加分 -(1) According to the enrollment form to establish the corresponding graph (2) find the ordered sequence of at least one topology (3) If all the topology to achieve an orderly sequence obtai