Find shortest paths to all vertices reachable from a given source vertex s.
Returns true iff there is no negative-weight cycle reachable from the source,
false if there is such a cycle. In case of a negative-weight cycle, the return
value of false indicates that there is no solution to the shortest path problem.
s - source vertex from which shortest paths are to be determined
true iff there is no negative-weight cycle reachable from s