line  1: 0    -> 50 is the root
line  3: l    -> 30 is the left child of 50
line  5: l    -> 20 is the left child of 30
line  7: ur   -> up 1 to 30, 40 is its right child
line  9: uur  -> up 2 to 50, 70 is its right child
nodes: 5, height: 3
preorder : 50 30 20 40 70
inorder  : 20 30 40 50 70
postorder: 20 40 30 70 50
sideways (root on the left, right children above):
      70
50
            40
      30
            20
