AVL Tree is: • A Binary Search Tree, • in which the heights of the left and right subtrees of the root differ by at most 1, and • the left and right subtrees are again AVL trees. The name comes from the discoverers of this method, 'son-Vel'skii and . The method dates from 1962. .Balance factor Balance factor: • left_higher: HL = HR + 1 • equal_height: • right_higher: