A binary search tree (BST) is a binary tree data structure which has the following properties:

->each node has a value;
->a total order is defined on these values;
->the left subtree of a node contains only values less than the node's value;
->the right subtree of a node contains only values greater than or equal to the node's value.


An AVL tree 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, therefore it is also called height-balanced. Lookup, insertion, and deletion all take O(log n) time in both the average and worst cases. Additions and deletions may require the tree to be rebalanced by one or more tree rotations.