Function encode

Convert a tree with an order to its prüfer code.

Tree requirements:

1. It is a branch and not a leaf. I.E: at least two nodes.
2. Root node value is minimum of all tree values. For example a tree of naturals should have the number 1 as its root node.

We call filterMinLeaf and get back:

1. Our tree with its minimal leaf removed
2. The parent of this removed leaf, or none if root

If the node has a parent, we add it to the left of the accumulated prüfer code array and recurse again on the now smaller tree.

When a node has no parent, we stop the recursion.