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Fig. 1: AVL tree with balance factors (green) In computer science, an AVL tree (named after inventors Adelson-Velsky and Landis) is a self-balancing binary search tree. In an AVL tree, the heights of the two child subtrees of any node differ by at most one; if at any time they differ by more than one, rebalancing is done to restore this property.
The tree rotation renders the inorder traversal of the binary tree invariant. This implies the order of the elements is not affected when a rotation is performed in any part of the tree. Here are the inorder traversals of the trees shown above: Left tree: ((A, P, B), Q, C) Right tree: (A, P, (B, Q, C))
Most operations on a binary search tree (BST) take time directly proportional to the height of the tree, so it is desirable to keep the height small. A binary tree with height h can contain at most 2 0 +2 1 +···+2 h = 2 h+1 −1 nodes. It follows that for any tree with n nodes and height h: + And that implies:
This rotation assumes that X has a right child (or subtree). X's right child, R, becomes X's parent node and R's left child becomes X's new right child. This rotation is done to balance the tree; specifically when the right subtree of node X has a significantly (depends on the type of tree) greater height than its left subtree.
A rotation operates on two nodes x and y, where x is the parent of y, and restructures the tree by making y be the parent of x and taking the place of x in the tree. To free up one of the child links of y and make room to link x as a child of y , this operation may also need to move one of the children of y to become a child of x .
To split a tree into two trees, those smaller than key x, and those larger than key x, we first draw a path from the root by inserting x into the tree. After this insertion, all values less than x will be found on the left of the path, and all values greater than x will be found on the right.
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The weak AVL tree is defined by the weak AVL rule: Weak AVL rule: all rank differences are 1 or 2, and all leaf nodes have rank 0. Note that weak AVL tree generalizes the AVL tree by allowing for 2,2 type node. A simple proof shows that a weak AVL tree can be colored in a way that represents a red-black tree.