说明：  本程序用来求有向网络中的最大流和最小费用的问题。这里最小费用是在可能有多个最大流的基础上优化得到的。故前提条件是最大流，如果没有没有最大流，可以不运输，成本为0，那就没有意义了。
(This program used to beg for the maximum flow to the network and the minimum cost. Here is the minimum cost in May have multiple maximum flow based on optimization get. So premise condition is maximum flow, if no no maximum flow, can not transport, the cost of 0, that is no sense. )