Our implementation of shortest path lengths in Code Fragment 14.13 relies on use of “infinity” as a numeric value, to represent the distance bound for vertices that are not (yet) known to be reachable...

Our implementation of shortest path lengths in Code Fragment 14.13 relies

on use of “infinity” as a numeric value, to represent the distance bound

for vertices that are not (yet) known to be reachable from the source.

Reimplement that function without such a sentinel, so that vertices, other

than the source, are not added to the priority queue until it is evident that

they are reachable.