edgesBetweenHash property
When displaying the edge, the direction of the edge is not considered, only the two nodes of the edge are considered, so a hash table is used here to store the two nodes of the edge for calculating the index of the edge, in a multi-graph with the same two nodes, the position of the edge can be determined according to the subscript.
在显示边时，不考虑边的方向，只考虑边的两个节点， 所以这里使用了一个哈希表来存储边的两个节点，以便计算边的索引， 在相同两个节点的多边图中，可以根据下标决定边的位置。
Implementation
Map<String, List<Edge>> edgesBetweenHash = {};