umber of simple shorest paths from a source node s to each node.
The input is a directed graph G=(V,E) as input, with edge weights
W:E-->Z (negative weights are possible).  The output is the number of
shortest paths (not necessarily disjoint) from s to each vertex u in
the graph.  If the graph has a negative or zero cycle, reachable from s,
then print a message "Non-positive cycle in graph.  DAC is not applicable".