1. Binary Trees Like a list, a (binary) tree can be empty or non-empty. In class we will explore a state-based implementation, similar to the LRS You are expected to read about alternate implementation strategies in the textbook

2. Method of a Binary Recursive Structure (BRS) method to grow tree: insertRoot method to shrink tree: removeRoot accessor/mutator pair for root: setRoot/getRoot accessor/mutator pair for left: setLeft/getLeft accessor/mutator pair for right: setRight/getRight visitor support: execute

3. State transitions Empty  NonEmpty: insertRoot on an empty tree NonEmpty  Empty: removeRoot from a tree that is a leaf –A leaf is a tree both of whose children are empty. No other state transitions can occur.

4. Moreover… insertRoot throws an exception if called on a nonEmpty tree removeRoot throws an exception if called on a non-leaf tree Isn’t this very restrictive?

5. Yes…but It is very restrictive, but for good reasons: –What would it mean to add a root to a tree that already has one? –If you could remove the root from a tree with nonEmpty children, what would become of those children?

6. Usage of a BRS A BRS is not used “raw”, but is used to implement more specialized trees. Consider how we defined the SortedList in terms of the LRS: by composition. We will now explore how to define a sorted binary tree (a Binary Search Tree) by composition with a BRS.

7. Binary Search Tree (BST) A binary search tree (BST) is a binary tree which maintains its elements in order. The BST order condition requires that, for every non-empty tree, the value at the root is greater than every value in the tree’s left subtree, and less than every value in the tree’s right subtree.

8. Example 1 Does this tree satisfy the BST order condition? Fred WilmaBetty BarneyPebbles

9. No! Barney Fred < Wilma Fred WilmaBetty BarneyPebbles

10. Example 2 Does this tree satisfy the BST order condition? Fred WilmaBetty BarneyPebbles

11. Yes! Barney < Betty < Fred < Pebbles < Wilma Fred WilmaBetty BarneyPebbles

12. Example 3 Does this tree satisfy the BST order condition? Fred WilmaBetty BarneyPebbles

13. No! Betty > Barney < Fred < Pebbles < Wilma Fred WilmaBetty BarneyPebbles

14. Example 4 …but if we swap Betty & Barney, we restore order! Fred Wilma Betty Barney Pebbles

15. Operations on a BST public BST insert(Comparable item) public BST remove(Comparable item) public boolean member(Comparable item) How do we support these? They don’t exist as methods on the BRS.

16. Visitors to the rescue! Visitors on the BRS have the same basic structure as on the LRS: they deal with two cases: –public Object emptyCase(BRS host, Object inp) –public Object nonEmptyCase(BRS host, Object inp)

17. Example: toStringVisitor public class ToStringVisitor extends IAlgo { public static final ToStringVisitor SINGLETON = new ToStringVisitor(); private ToStringVisitor() {} public Object emptyCase(BRS host, Object input) { return "[]"; } public Object nonEmptyCase(BRS host, Object input) { return "[" + host.getLeft().execute(this, null) + " " + host.getRoot().toString() + " " + host.getRight().execute(this, null) + "]"; }

18. Back to the BST Let’s first consider the insert operation. We must consider two possibilities: –the underlying BRS is empty just insertRoot –the underlying BRS is nonEmpty we can’t insertRoot, because the BRS is nonEmpty compare new item with root – determine whether new item belongs in left or right subtree – insert recursively into correct subtree

19. A first cut at the visitor public class InsertVisitor extends IAlgo { public Object emptyCase(BRS host, Object item) { return host.insertRoot(item); } public Object nonEmptyCase(BRS host, Object item) { if ( ((Comparable) item).compareTo(host.getRoot()) < 0 ) { // item belongs in left subtree return host.getLeft().execute(this, item); } else if ( ((Comparable) item).compareTo(host.getRoot()) > 0 ) { // item belongs in right subtree return host.getRight().execute(this, item); } else {// item is already in tree (Note UNIQUENESS ASSUMPTION) return host; }

20. How about determining membership for an item? We must consider two possibilities: –the underlying BRS is empty just item was not found –the underlying BRS is nonEmpty compare new item with root – determine whether new item has been found, or whether it would be in left or right subtree – look recursively into correct subtree

21. A first cut at the visitor public class MemberVisitor extends IAlgo { public Object emptyCase(BRS host, Object item) { return new Boolean(false); } public Object nonEmptyCase(BRS host, Object item) { if ( ((Comparable) item).compareTo(host.getRoot()) < 0 ) { // item belongs in left subtree return host.getLeft().execute(this, item); } else if ( ((Comparable) item).compareTo(host.getRoot()) > 0 ) { // item belongs in right subtree return host.getRight().execute(this, item); } else {// item is already in tree (Note UNIQUENESS ASSUMPTION) return new Boolean(true); }

22. Did you notice similarity? The structure of the insert and membership visitors was the same. A small number of details differed. Let’s unify!

23. The find visitor public class Find extends IAlgo { public Object emptyCase(BRS host, Object item) { return host; } public Object nonEmptyCase(BRS host, Object item) { if ( ((Comparable) item).compareTo(host.getRoot()) < 0 ) { // item belongs in left subtree return host.getLeft().execute(this, item); } else if ( ((Comparable) item).compareTo(host.getRoot()) > 0 ) { // item belongs in right subtree return host.getRight().execute(this, item); } else {// item is already in tree (Note UNIQUENESS ASSUMPTION) return host; }

24. Look at BST implementation insert, remove and member ALL make use of the same Find visitor: –first find the insertionPoint, –then do the right thing.

25. The insert method public BST insert(Comparable item) { BRS insertPoint = (BRS) _tree.execute(new Find(), item); only insert item into insertPoint if empty (duplicates ignored) return this; }

26. The insert method public BST insert(Comparable item) { BRS insertPoint = (BRS) _tree.execute(new Find(), item); insertPoint.execute(new IAlgo() { public Object emptyCase(BRS host, Object input) { host.insertRoot(input); return null; } public Object nonEmptyCase(BRS host, Object input) { return null; } }, item); return this; }

27. The member method public boolean member(Comparable item) { BRS insertPoint = (BRS) _tree.execute(new Find(), item); return whether insertPoint is empty or nonEmpty }

28. The member method public boolean member(Comparable item) { BRS insertPoint = (BRS) _tree.execute(new Find(), item); return ((Boolean) insertPoint.execute(new IAlgo() { public Object emptyCase(BRS host, Object input) { return new Boolean(false); } public Object nonEmptyCase(BRS host, Object input) { return new Boolean(true); } }, null)).booleanValue(); }

29. The remove method public BST remove(final Comparable item) { BRS removePoint = (BRS) _tree.execute(new Find(), item); handle removal differently in five different cases: empty tree non-empty tree with both children empty (leaf case) non-empty tree with left child empty, right child non-empty non-empty tree with left child non-empty, right child empty non-empty tree with both children non-empty return this; }

30. The remove method public BST remove(final Comparable item) { BRS removePoint = (BRS) _tree.execute(new Find(), item); removePoint.execute(new FiveStateVisitor() { private IAlgo theRemovalVisitor = this; public Object emptyCase(BRS host, Object input) { return host; } public Object leafCase(BRS host, Object input) { host.removeRoot(); return host; } public Object leftNonEmptyCase(BRS host, Object input) { host.setRoot(host.getLeft().getRoot()); host.setRight(host.getLeft().getRight()); host.setLeft(host.getLeft().getLeft()); return host; } public Object rightNonEmptyCase(BRS host, Object input) { host.setRoot(host.getRight().getRoot()); host.setLeft(host.getRight().getLeft()); host.setRight(host.getRight().getRight()); return host; } public Object leftRightNonEmptyCase(BRS host, Object input) { Object smallestInRight = host.getRight().execute(new IAlgo() { public Object emptyCase(BRS host, Object parent) { Object smallest = ((BRS) parent).getRoot(); ((BRS) parent).execute(theRemovalVisitor, null); return smallest; } public Object nonEmptyCase(BRS host, Object parent) { return host.getLeft().execute(this, host); } }, null); host.setRoot(smallestInRight); return host; } }, item); return this; }

31. Question How do we make the 5-way distinction? See the FiveStateVisitor definition in the lecture code repository. Basic idea: check left, then check right.