each town struct, we add
  int distance
  bool visited

set all distances to inf
start.distance = 0;
Q.enqueue(start);
while Q.not empty()
{ here = Q.dequeue();
  here.visited = true
  if here == dest break;
  for i = 0 to #exits-1
    road r = here.exits[i];
    town there = r.goesto
    if there.distance != inf continue
    there.distance = here.distance + 1
    Q.enqueue(there)

-------------------------------------------
Data Structure = a Priority Queue
  conatins items (anything/towns)
+ estimated time of arrival (ETA)

Operations:
  create empty (constructor)
  add new item with its priority
  change priority of existing item
  remove item with lowest ETA

start.ETA = 0
PQ.add(start)
while (true)
  here = PQ.remove_smallest()
  here.visited = true
  if (here==dest) break;
  for i = 0 to #EXITS-1
    r = here.exit[i]
    there = r.goesto
    if there.visited continue
    timethere = here.eta + r.length
    if there.eta = inf
      PQ.add(there)
    else if timethere < there.eta
      PQ.update(there, timethere);

PQ implementations
------------------
Linked list (unordered)
  remove_smallest =
    search entire LL for item with min ETA, O(N)
  add = insert wherever is easiest, O(1)
  update = search entire LL for item, change its ETA, O(N)

Linked list (ordered by ETA, smallest first)
  remove_smallest = remove from front, O(1)
  add = search for right place and add there, O(N)
  update = search for it, change eta, and re-add it, O(N)

Tree ordered by ETA
  remove_smallest = search, delete, O(log N)
  add = normal tree insertion, O(log N)
  update = search whole tree to find town, O(N)

Adding to a heap
----------------
A, eta = 55
B, eta = 68
C, eta = 40
D, eta = 72
E, eta = 45
F, eta = 21



                       F21

             E45                A55

        D50       E47       B68      C58

     G52   H66

real way to remove smallest
  swap smallest(top) with last (bottom right)
  remove last

                       H66

             E45                A55

        D50       E47       B68      C58

     G52  

Moved thing is at the top
if moved thing has > eta than top of either subtree
  swap them
  and repeat