VK Cup 2017

http://codeforces.com/problemset/problem/770/C

题目大意：
一共有N个科目，其中有M个主要科目，只要M个主要科目都通过了，那么对应主人公就可以毕业了，也就是达成了目的。
现在有一个列表，表示要通过第i门课程需要先通过的科目列表。现在主人公希望得到一个通过科目的顺序，使得最终他通过最少的科目使得主人公毕业。

做法：第一感觉像是拓扑排序，毕竟有个事情的先后顺序，冷静分析一下，好像不断dfs就好了，对于每个重要的科目，我先标记他为1，代表这个科目

有个前置科目要先学，如果dfs到一层，这个科目的前置科目要先学，这个前置科目的前置科目（也就是前前置科目），如果有重叠那么就永远学不到了，

那么就是-1，对于正常dfs到下层，标记为2就好了，就把这俩分开了。

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