An AVL tree is a type of binary search tree that is always in a state of partially balanced.
The balance is measured as a difference between the heights of the subtrees from the root.
This self-balancing tree was known to be the first data structure to be designed as such.
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.
