If we index the nodes of the tree as 1 for the top, 2--3 for the next
horizontal row, 4--7 for the next, ...then the parent-child
traversal becomes particularly easy.
The basic idea is that the rows of the tree start at indices 1, 2, 4, ....
BinTree(13) yields the vector
[[8 4 9 2 10 5 11 1 12 6 13 3 7]] meaning that the smallest element
will be in position 8 of the tree, the next smallest in position 4, etc.