An **AVL tree** is a self balancing binary search tree, and it was the first such data structure to be invented. In an AVL tree, the heights of the two child subtrees of any node differ by at most one. 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 **balance factor** of a node is the height of its left subtree minus the height of its right subtree (sometimes opposite) and a node with balance factor 1, 0, or −1 is considered balanced. A node with any other balance factor is considered unbalanced and requires rebalancing the tree. The balance factor is either stored directly at each node or computed from the heights of the subtrees.

AVL trees are often compared with red-black trees because they support the same set of operations and because red-black trees also take O(log *n*) time for the basic operations. Because AVL trees are more rigidly balanced, they are faster than red-black trees for lookup intensive applications. However, it looks like red-black trees could be faster for insertion and element removal.

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 preceded or followed by one or more operations called tree rotations, which help to restore the height balance of the subtree.

**Lookup**

Lookup in an AVL tree is performed exactly as in an unbalanced binary search tree. Because of the height-balancing of the tree, a lookup takes O(log *n*) time. No special actions need to be taken, and the tree’s structure is not modified by lookups. If each node additionally records the size of its subtree (including itself and its descendants), then the nodes can be retrieved by index in O(log *n*) time as well.

**Insertion**

After inserting a node, it is necessary to check each of the node’s ancestors for consistency with the rules of AVL. For each node checked, if the balance factor remains −1, 0, or +1 then no rotations are necessary. However, if the balance factor becomes ±2 then the subtree rooted at this node is unbalanced. If insertions are performed serially, after each insertion, at most one of the following cases needs to be resolved to restore the entire tree to the rules of AVL.

There are four cases which need to be considered, of which two are symmetric to the other two. Let P be the root of the unbalanced subtree, with R and L denoting the right and left children of P respectively.

**Right-Right case** and **Right-Left case**:

- If the balance factor of P is -2 then the right subtree outweights the left subtree of the given node, and the balance factor of the right child (R) must be checked. The left rotation with P as the root is necessary.

- If the balance factor of R is -1, a
**double left rotation**(with respect to P and then R) is needed (Right-Right case).

- If the balance factor of R is +1, two different rotations are needed. The first rotation is a
**right rotation**with R as the root. The second is a**left rotation**with P as the root (Right-Left case).

**Left-Left case** and **Left-Right case:**

- If the balance factor of P is +2, then the left subtree outweighs the right subtree of the given node, and the balance factor of the left child (L) must be checked. The right rotation with P as the root is necessary.
- If the balance factor of L is +1, a
**double right rotation**(with respect to P and then L) is needed (Left-Left case). - If the balance factor of L is -1, two different rotations are needed. The first rotation is a
**left rotation**with L as the root. The second is a**right rotation**with P as the root (Left-Right case). The pictorial representation is given below:

I have done the implementation of AVL tree in C.

The program for rotation is shown below:

For left-right,

For left-left,

For right-left,

For right-right,

The complete code of implementation of AVL tree in C is available here: https://bitbucket.org/vidyakv/avl-tree/changeset/a112ab2dd2f4

**Deletion**

If the node is a leaf or has only one child, remove it. Otherwise, replace it with either the largest in its left subtree (inorder predecessor) or the smallest in its right subtree (inorder successor), and remove that node. The node that was found as a replacement has at most one subtree. After deletion, retrace the path back up the tree (parent of the replacement) to the root, adjusting the balance factors as needed.

Note: The image of tree used in this post is taken from wikipedia.