AVL tree - 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 said to be height-balanced. 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.

Psuedo code:

IF tree is right heavy
{
  IF tree's right subtree is left heavy
  {
     Perform Double Left rotation
  }
  ELSE
  {
     Perform Single Left rotation
  }
}
ELSE IF tree is left heavy
{
  IF tree's left subtree is right heavy
  {
     Perform Double Right rotation
  }
  ELSE
  {
     Perform Single Right rotation
  }
}

Reference:
http://en.wikipedia.org/wiki/AVL_tree
http://oopweb.com/Algorithms/Documents/AvlTrees/Volume/AvlTrees.htm