AVL Tree 1

is a binary search tree that For each node: –The height of its left and right subtree can only differ at most 1. –If a tree is an empty tree, we let its height == -1. The height of an AVL tree is log N –The exact figure is 1.44log(N+2)-.328 The height difference constraint leads to: –The minimal number of nodes for a tree of height h, s(h) = s(h-1)+s(h-2)+1 When h=0, s(h) =1 When h=1, s(h) =2 –This is similar to fibonacci. 2

AVL Non-AVL

Various functions All functions are O(log(N)), except insert and remove -> these may destroy the properties of AVL. Inserting a new node here destroys AVL

Fix insert+ remove with rotation insert –Only nodes on the path from the insertion point to the root have a chance of height change. –If we follow up from the inserted node, we will eventually find the deepest node that loses its AVL property. We will fix such node. There can only be 4 types of such node. 5

The 4 types: Solve by single rotation Solve by double rotation 6

Single rotation after insert a b X Y Z Pull up a b X YZ Move the attachment place to the problem node. If it is fixed now, no further rotation is needed. 7

example This node loses AVL property

Example 2 Starting from nothing, we insert 3,2,1 loses AVL To be continued rotate 9

Then we insert 4, loses AVL To be continued

Then we insert loses AVL rotate To be continued 11

Then we insert loses AVL rotate

Double rotation When Single rotation is useless a b Y X Z a b Y X Z Y does not change level 13

a b Y X Z c Insertion takes place. There must be something in. a b X Z Y1Y2 Loses AVL 14

The first rotation c a b X Z Y1Y2 c a b X Z Y1 Y2 15

the second rotation c a b X Z Y1 Y2 c b X a Z Y1 Y2 16

We do not know whether Y1 or Y2 is the cause. But it does not matter because in the end both of them reduce in level. It is like letting c release Y1,Y2 and carry a and b instead. c a b X Z Y1Y2 c a b X Z Y1Y2 Y1, Y2 find places to attach 17

example Insert 16,15: loses AVL when inserting Loses AVL rotate

Insert Loses AVL Must jump rotate 19

Insert 13 Loses AVL Two right moves: single rotation rotate 20

Insert 12: this is an obvious single rotation Loses AVL rotate 21

Insert 11: another single rotation Loses AVL rotate

Insert 10: another single rotation Loses AVL rotate

Insert 8 Insert 9: simple double rotation Loses AVL

1. class AvlNode 2. { 3. // Constructors 4. AvlNode( Comparable theElement ) 5. { 6. this( theElement, null, null ); 7. } 8. AvlNode( Comparable theElement, AvlNode lt, AvlNode rt ) 9. { 10. element = theElement; 11. left = lt; 12. right = rt; 13. height = 0; 14. } 15. // Friendly data; accessible by other package routines 16. Comparable element; // The data in the node 17. AvlNode left; // Left child 18. AvlNode right; // Right child 19. int height; // Height 20. } There will be a method that returns height. So we do not have to worry if the node we are looking at is null. 25

1. public class AvlTree 2. { private AvlNode root; 3. /** 4. * Construct the tree. 5. */ 6. public AvlTree( ) 7. { 8. root = null; 9. } 10. /** 11. * Insert into the tree; duplicates are ignored. 12. x the item to insert. 13. */ 14. public void insert( Comparable x ) 15. { 16. root = insert( x, root ); 17. } 26

18. /** 19. * Find the smallest item in the tree. 20. smallest item or null if empty. 21. */ 22. public Comparable findMin( ) 23. { 24. return elementAt( findMin( root ) ); 25. } 26. /** 27. * Find the largest item in the tree. 28. the largest item of null if empty. 29. */ 30. public Comparable findMax( ) 31. { 32. return elementAt( findMax( root ) ); 33. } Same as binary search Tree 27

1. /** 2. * Return the height of node t, or -1, if null. 3. */ 4. private static int height( AvlNode t ) 5. { 6. return t == null ? -1 : t.height; 7. } 8. /** 9. * Return maximum of lhs and rhs. 10. */ 11. private static int max( int lhs, int rhs ) 12. { 13. return lhs > rhs ? lhs : rhs; 14. } Helper functions 28

1. /** 2. * Internal method to insert into a subtree. 3. x the item to insert. 4. t the node that roots the tree. 5. the new root. 6. */ 7. private AvlNode insert( Comparable x, AvlNode t ) 8. { 9. if( t == null ) 10. t = new AvlNode( x, null, null ); 11. else if( x.compareTo( t.element ) < 0 ) 12. { 13. t.left = insert( x, t.left ); 14. if( height( t.left ) - height( t.right ) == 2 ) 15. if( x.compareTo( t.left.element ) < 0 ) 16. t = rotateWithLeftChild( t ); 17. else 18. t = doubleWithLeftChild( t ); 19. } 20. Insert If = =2 is true, x is below. t 29

21. else if( x.compareTo( t.element ) > 0 ) 22. { 23. t.right = insert( x, t.right ); 24. if( height( t.right ) - height( t.left ) == 2 ) 25. if( x.compareTo( t.right.element ) > 0 ) 26. t = rotateWithRightChild( t ); 27. else 28. t = doubleWithRightChild( t ); 29. } 30. else 31. ; // Duplicate; do nothing 32. t.height = max( height( t.left ), height( t.right ) ) + 1; 33. return t; 34. } t 30

1. /** 2. * Rotate binary tree node with left child. 3. * For AVL trees, this is a single rotation for case * Update heights, then return new root. 5. */ 6. private static AvlNode rotateWithLeftChild( AvlNode k2 ) 7. { 8. AvlNode k1 = k2.left; 9. k2.left = k1.right; 10. k1.right = k2; 11. k2.height = max( height( k2.left ), height( k2.right ) ) + 1; 12. k1.height = max( height( k1.left ), k2.height ) + 1; 13. return k1; 14. } k2 k1 k2 k1 31

1. /** 2. * Rotate binary tree node with right child. 3. * For AVL trees, this is a single rotation for case * Update heights, then return new root. 5. */ 6. private static AvlNode rotateWithRightChild( AvlNode k1 ) 7. { 8. AvlNode k2 = k1.right; 9. k1.right = k2.left; 10. k2.left = k1; 11. k1.height = max( height( k1.left ), height( k1.right ) ) + 1; 12. k2.height = max( height( k2.right ), k1.height ) + 1; 13. return k2; 14. } k1 k2 k1 k2 32

1. /** 2. * Double rotate binary tree node: first left child 3. * with its right child; then node k3 with new left child. 4. * For AVL trees, this is a double rotation for case * Update heights, then return new root. 6. */ 7. private static AvlNode doubleWithLeftChild( AvlNode k3 ) 8. { 9. k3.left = rotateWithRightChild( k3.left ); 10. return rotateWithLeftChild( k3 ); 11. } k2 k1 X Y1Y2 k3 Z k2 k3 k1 X Z Y1Y2 k2 k1 X Y1 Y2 k3 Z 33

1. /** 2. * Double rotate binary tree node: first right child 3. * with its left child; then node k1 with new right child. 4. * For AVL trees, this is a double rotation for case * Update heights, then return new root. 6. */ 7. private static AvlNode doubleWithRightChild( AvlNode k1 ) 8. { 9. k1.right = rotateWithLeftChild( k1.right ); 10. return rotateWithRightChild( k1 ); 11. } k2 k1 k3 X Z Y1Y2 k2 k3 X Y1 Y2 k1 Z k2 k3 X Y1 Y2 k1 Z 34

Remove a node from AVL First, do a normal remove Then rebalance, starting from the node just above the removed node up to the root. We need to rebalance every node because balancing a child can make its parent loses AVL. 35

Loses AVL 36

private AVLNode basicRemove( Comparable x, AVLNode t ) { if( t == null ) return t; // Item not found; do nothing if( x.compareTo( t.element ) < 0 ) t.left = basicRemove( x, t.left ); else if( x.compareTo( t.element ) > 0 ) t.right = basicRemove( x, t.right ); else if( t.left != null && t.right != null ) // Two children { t.element = findMin( t.right ).element; t.right = basicRemove( t.element, t.right ); } else t = ( t.left != null ) ? t.left : t.right; t.setHeight(); return t; } This is the same as BinaryNode’s remove() 37

private AVLNode remove(Comparable x, AVLNode r){ r = basicRemove(x,r); r = rebalance(r); return r; } public void remove(Comparable x){ root = remove(x, root); } private void setHeight(){// this is in AVLNode height = 1 + Math.max(height(left), height(right)); } private static int height(AVLNode n){ // this is in AVLNode return (n == null ? -1 : n.height); } 38

private AVLNode rebalance(AVLNode r){ if (r == null) return r; int balance = balanceValue(r); if (balance == 2){//left hand side has more nodes than it should have if (balanceValue(r.left) ==-1) r.left = rotateWithRightChild(r.left); r = rotateWithLeftChild(r); } else if (balance == -2){//right hand side has more nodes than it should have if (balanceValue(r.right) ==1) r.right = rotateWithLeftChild(r.right); r = rotateWithRightChild(r); } r.setHeight(); return r; } public int balanceValue(AvlNode t){ return height(t.left) – height(t.right); } 39