Tree operations so far have involved just one tree.

An operation on two trees:   t = joinTrees(t1,t2)

• Pre-conditions:
  • takes two BSTs; returns a single BST
  • max(key(t1)) < min(key(t2))
• Post-conditions:
  • result is a BST (i.e. correctly ordered)
  • containing all items from t1 and t2