AVL Trees Balanced Trees. AVL Tree Property A Binary search tree is an AVL tree if : –the height of the left subtree and the height of the right subtree.

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AVL Trees Balanced Trees

AVL Tree Property A Binary search tree is an AVL tree if : –the height of the left subtree and the height of the right subtree differ at most by one, and –the left and right subtrees are also AVL trees The next few slides show AVL trees if different heights which have the fewest number of nodes possible

AVL - height 1

AVL - height 2

AVL - height 3

AVL - height 4

AVL - height 5

AVL - height 6

AVL - height 7

AVL - height 8

AVL - height 9

AVL - height 10

Counting nodes

Relationship to Fibonacci Let N be the fewest number of nodes in an AVL tree of height H It is straightforward to show that N = F(H+3) - 1, where F(k) is the k th Fibonacci number For large values of k,

number of nodes The fewest number of nodes in an AVL tree with height H is given by

Solving for H if we solve this near equality for H, we get H  1.44 log 2 N This means that the height of an AVL tree with N nodes is no more than 44% larger than the optimal height of a binary search tree with N nodes

Building an AVL tree During the building of an AVL tree, the only time a node can possibly get out of balance is when a new node is inserted. Our attention is on the ancestor closest to the newly inserted node which has become unbalanced There are basically two cases

The “outie” case r x T1 T2T3 n r - the nearest ancestor which is out of balance n - the newly inserted node height of T1, T2, and T3 are all the same, say h

single rotation r x T1 T2T3 n r x T1T2 T3 n

After the rotation x is now the root the height of the tree is the same as it was before inserting the node, so no other ancestor is unbalanced the root x is balanced r x T1T2 T3 n

The “innie” case r is the nearest out-of- balance ancestor T1 and T4 have height h T2 and T3 have height h-1 n is the newly inserted node - either in T2 or T3 r w T1 T2T3 n x T4

Double Rotation r w T1 T2T3 n x T4 r w T1 T2T3 n x T4

After the Rotation w is now the root with left child r and right child x The height of the tree is the same as before the insertion, so no other ancestor is now out-of-balance This tree is balanced r w T1 T2T3 n x T4

The other rotations These two demonstrations show the Single Left rotation and the Double Left rotation (used when the nearest out-of-balance ancestor is too heavy on the right) Similar rotations are performed when the nearest out-of-balance ancestor is heavy on the left -- these are called Single Right and Double Right Rotations

Deletion from an AVL Tree Deletion of a node from an AVL tree requires the same basic ideas, including single and double rotations, that are used for insertion The steps are on the next slide

Steps in deleting X reduce the problem to the case where X has only one child Delete the node X. The height of the subtree formerly rooted at X has been reduced by one We must trace the effect on the balance from X all the way back to the root until we reach a node which does not need adjustment