poj上网络流的几道题（1149，1689，2112，2391，2455，3436），关键是抽象出网络流的模型，这样才能想出怎么运用他。-poj the network flow a few questions (1149,1689,2112,2391,2455,3436), the key is to abstract the network flow model, in order to figure out how to use him.