LayerEdge InEdgeOfVirtualNode(int v)
 {
     return(this.BT ? graph.InEdgeOfVirtualNode(v) : graph.OutEdgeOfVirtualNode(v));
 }
 //here u is a virtual vertex
 private LayerEdge IncomingEdge(int u)
 {
     return(layeredGraph.InEdgeOfVirtualNode(u));
 }