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AVL Tree Example

CS 402 Advanced Tree Examples

Here is an example of insertion into an AVL tree. In each figure, we give the balance factor for each node in red, and its data value in black. Starting with an empty tree, we insert the keys 14, 71, 3, 52, 68, 92, 59, 37, 22, 49 and 41. The sequence begins here, or you may use the index below.

The steps for inserting into an AVL tree are these:

  1. Insert the key as you would in an ordinary BST. The new node will have factor 0. The path you take as you move from the root looking for the place to insert the node is the insertion path. As you search down the tree, remember the location of the last node which has a balance factor ±1. Call this node A. It is the node on the insertion path closest to the insertion point having a non-zero balance factor.

  2. Adjust each balance factor of each node along the search path from below A to the new node, excluding the endpoints. Each will be initially zero. If you move left when leaving the node, set its factor to +1. If you move right, set to -1. If there is no A node, adjust the factors for all the nodes on the search path starting with the root up to the new node, and omit the remaining steps.

  3. Adjust the the balance factor of A. If you move right to the insertion, subtract one, if you move left, add one. This will leave A's factor at -2, 0, or +2. If it is 0, the insertion is complete.

  4. If A has factor ±2, perform the appropriate rotation on the subtree rooted at A. The rotation specifies new balance factors for the A, B, and C nodes. No other balance factors are changed by the rotation.

Beginning with the empty tree, we present the tree after each of these steps:

o After Inserting 14
o After Inserting 71
o After Inserting 3
o After Inserting 52
o After Inserting 68
o After Inserting 92
o After Inserting 59
o After Inserting 37
o After Inserting 22
o After Inserting 49
o After Inserting 41