Change EXTRACT-MIN in Dijkstra's to pick out the node u
with the smallest value of d[u]+h(u). [Note, at the smallest value
of d[u]+h(u), 
because of
monotonicity.]
Idea:
- The quantity f(u) is an underestimate of the total distance from
s to t through u (true distance from s to u plus an
underestimate of the distance from u to t).
- By focussing the search on the nodes with the smallest value of
f(u), we are likely to be doing work along the true shortest path.
This is called A 
.
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