实现了所选课程的正确排序，由于部分课程有先修课程，因此用到了拓扑排序的知识。-Selected the correct course to achieve the sort, as some courses have pre-course, so use a topological sort of knowledge.

选课的MFC软件设计，基于拓扑排序，能够一步一步的进行，一个个选择，只要你一看到MFC界面，你就马上知道怎么选的！-Elective MFC software design, based on the topological sort can be carried out step by step, one choice, as long as you see the MFC interface, you immediately know how to pick it!