IN-ORDER TRAVERSAL  (remember this)
 - Traverse left subtree
 - Visit root node
 - Traverse right subtree

POST-ORDER TRAVERSAL  (remember this)
 - Traverse left subtree
 - Traverse right subtree
 - Visit the root

PRE-ORDER TRAVERSAL  (remember this)
 - Visit the root
 - Traverse left subtree
 - Traverse right subtree