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AVL trees : ウィキペディア英語版
AVL tree

In computer science, an AVL tree (Georgy Adelson-Velsky and Evgenii Landis' tree, named after the inventors) is a self-balancing binary search tree. It was the first such data structure to be invented.〔Robert Sedgewick, ''Algorithms'', Addison-Wesley, 1983, ISBN 0-201-06672-6, page 199, chapter 15: Balanced Trees.〕 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. Lookup, insertion, and deletion all take O(log ''n'') time in both the average and worst cases, where ''n'' is the number of nodes in the tree prior to the operation. Insertions and deletions may require the tree to be rebalanced by one or more tree rotations.
The AVL tree is named after its two Soviet inventors, Georgy Adelson-Velsky and Evgenii Landis, who published it in their 1962 paper "An algorithm for the organization of information".〔 English translation by Myron J. Ricci in ''Soviet Math. Doklady'', 3:1259–1263, 1962.〕
AVL trees are often compared with red-black trees because both support the same set of operations and take O(log ''n'') time for the basic operations. For lookup-intensive applications, AVL trees are faster than red-black trees because they are more rigidly balanced. Similar to red-black trees, AVL trees are height-balanced. Both are in general not weight-balanced nor μ-balanced for any \scriptstyle \mu\leq\tfrac12;〔(AVL trees are not weight-balanced? (meaning: AVL trees are not μ-balanced?) )
Thereby: A Binary Tree is called \mu-balanced, with 0 \le\mu\leq\tfrac12, if for every node N, the inequality
: \tfrac12-\mu\le\tfrac\le \tfrac12+\mu
holds and \mu is minimal with this property. |N| is the number of nodes below the tree with N as root (including the root) and N_l is the left child node of N.〕 that is, sibling nodes can have hugely differing numbers of descendants.
==Operations==

Basic operations of an AVL tree involve carrying out the same actions as would be carried out on an unbalanced binary search tree, but modifications are followed by zero or more operations called tree rotations, which help to restore the height balance of the subtrees.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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