Problem:
 Input: a Digraph (directed graph G) and a vertex v
  each edge has an associated non negatie weight
 Output: for each w in Vertices(G), the cost of the path of minimal cost
  from v to w where the cost of a path is the sum of the weights of the
  edges in the path

Hints:
 1. BFS solves the problem when all weights are 1
 2. a queue is a priority queue where all priorities are the same (e.g. 1...)