Let G be the weighted directed graph whose nodes are the letters
'a','b','c','d','e','f','g','h', and whose edges are listed below.
(For example, there is an edge from 'a' to 'b' of weight 2.)

1.  Find a topological order of the nodes of G.
2.  Use that topological order to solve the single source minimum path
problem for G, where the source is 'a'.

a b  2
a g -1
b c  4
d e  0
d g -3
d h -1
g b  4
g c  1
g e  2
g f  3
e f -1
h e  2