摘要： 只能说是很好的一道题目~~1000pt似乎从来木有不好的。。。。学到的最重要的一点是：It is often a handy tool to formalize the conditions in the problem.还有，就是又学到了一个定理，关于二分图的： Hall's marriage theorem ——There is a perfect matching if and only if for each subset of the left side of the graph, the total number of neighbours of this subset i 阅读全文