# What Are AA Trees in Data Structure?

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Scott Campbell

An AA tree, also known as an Arne Andersson tree, is a self-balancing binary search tree that ensures optimal performance for various operations. It was introduced by Arne Andersson in 1989 as an improvement over other balanced search trees such as AVL trees and red-black trees.

## Structure of an AA Tree

An AA tree is composed of nodes that store key-value pairs. Each node has a key, a value, and two child pointers – left and right. In addition to these basic components, AA trees have an extra attribute called ‘level’ that helps maintain balance in the tree.

### Level Attribute

The ‘level’ attribute of a node represents the level of the node in the tree. Level 1 denotes the leaf nodes, while higher levels represent nodes closer to the root. The primary purpose of this attribute is to ensure that the difference in levels between any two siblings is at most one.

## Properties of AA Trees

AA trees have several properties that make them efficient and reliable:

• Property 1: Binary Search Tree Property – The keys in an AA tree’s left subtree are less than the key in its root node, while the keys in its right subtree are greater than or equal to the key in its root node.
• Property 2: Level Property – The level of any right child is always less than or equal to the level of its parent.
• Property 3: Right Child Property – No node has two consecutive right children with level equal to or greater than itself.
• Property 4: Null Path Property – All paths from a node to its descendant leaves have the same number of right children with level equal to itself.

## Operations on AA Trees

AA trees support various operations that are essential in any data structure:

• Insertion – To insert a new key-value pair into an AA tree, we follow a process similar to that of inserting into a regular binary search tree. After insertion, we perform rotations and adjustments to maintain the balance and level properties.
• Deletion – Deleting a node from an AA tree involves finding the corresponding node and removing it.

Similar to insertion, we also need to perform rotations and adjustments after deletion to restore the balance and level properties.

• Searching – The search operation in an AA tree is similar to that in a binary search tree. We compare the Target key with the keys at each node and traverse either left or right accordingly until we find a match or reach a leaf node.
• Traversal – AA trees support various traversal methods such as in-order, pre-order, and post-order traversal, which allow us to visit all the nodes in the tree in a specific order.