说明：  本例以由拓扑关系的arc/info 文件为数据源。其中a1,b1,c1是以fnode排序生成的数组，a1对应fnode,b1对应tnode,c1对应length,同样a2,b2,c2,是以tnode 生成的数组。Indexa1是对应某一起点与其相连的终点的个数，indexb1时对应某一终点与其相连的起点的个数，即其拓扑关系
(length, the same a2, b2, c2, is based on an array of tnode generated. Indexa1 is a starting point corresponding to the number of its connected to the end, indexb1 corresponds to an end when the starting point of the number of its connected, that is, its topological relationship)