- James S. Plank
- Directory:
**/home/plank/cs140/Notes/AVLTree** - Last Revision:
*Tue Apr 22 09:11:20 EDT 2014*

UNIX>That's a big problem with binary search trees. AVL trees (and other balanced trees like Splay trees, Red-Black trees, B-trees, 2-3 trees, etc) make sure that their trees are balanced so that the various operations are much faster. For example, the programwc input.txt10000 51456 770000 input.txt UNIX>head input.txtINSERT Ellie Warlike 944-867-2246 165-79-8849 INSERT David Bobble 026-631-5520 826-96-9094 INSERT Isaac Giuliano 462-055-3150 827-30-6292 INSERT Madison Fiend 193-149-4333 106-62-2934 INSERT Chloe Skew 257-554-8530 481-12-6340 INSERT Julia Postdoctoral 018-992-9715 512-23-5507 INSERT Connor Teledyne 808-602-6582 702-11-9340 INSERT Caleb Disciple 457-440-4397 076-91-9105 INSERT Avery Chloe Panther 243-649-0973 727-68-6107 INSERT Anna Placenta 193-082-7570 836-85-9844 UNIX>time sh -c "bstree_test - < input.txt"0.099u 0.006s 0:00.11 81.8% 0+0k 0+1io 0pf+0w UNIX>time bstree_test - < input.txt0.105u 0.004s 0:00.11 90.9% 0+0k 0+0io 0pf+0w UNIX>time sh -c "sort input.txt | bstree_test -"1.721u 0.014s 0:01.73 100.0% 0+0k 0+1io 0pf+0w UNIX>

UNIX>As you can see, since sorting takes .25 seconds, performing insertions with the AVL tree takes the same time when the input is sorted as when it is not sorted.time avltree_test - < input.txt0.099u 0.003s 0:00.10 90.0% 0+0k 0+0io 0pf+0w UNIX>time sh -c "sort input.txt | avltree_test -"0.336u 0.011s 0:00.34 100.0% 0+0k 0+0io 0pf+0w UNIX>time sh -c "sort input.txt > /dev/null"0.245u 0.007s 0:00.25 96.0% 0+0k 0+0io 0pf+0w UNIX>

A central operation with AVL Trees is a *rotation*. It is a way of
changing a binary search tree so that it remains a binary search tree,
but changes how it is balanced. The concept is illustrated below:

**B** and **D** are nodes in a binary search tree. They can occur anywhere
a tree, but we don't worry about what's above them -- just what's below them.
**A**, **C** and **E** are subtrees that rooted as the children of
**B** and **D**. They may be empty. If they are not empty, then since
the tree is a binary search tree, we know that:

- Every node in
**A**has a key less than**B**'s key. - Every node in
**C**has a key less than**D**'s key, but greater than**B**'s key. - Every node in
**E**has a key greater than**D**'s key.

When we perform a rotation, we perform it *about* a node. For example,
the rotation pictured above rotates about node **D** to turn the tree on the
left to the tree on the right. It also shows that you can turn the tree on the
right to the tree on the left by rotating about node **B**.

When you rotate about a node, you are going to change the tree so that the
node's parent is now the node's child. The middle subtree (subtree **C**)
will change from being one node's child to being the other node's child.
The rotation does not violate any of the properties of binary search trees.
However, it changes the shape of the tree, and there are multiple types
of trees, like AVL, Splay and Red-Black trees that
employ rotations to ensure that the trees are balanced.

Below are some example rotations. Make sure you understand all of them:

The definition of an AVL tree is follows:

- Every subtree of the tree is an AVL tree.
- For each node, the difference between the heights of its left and right subtrees is at most one.

And below are two trees that are binary search trees, but are not AVL trees.

Binky violates the definition |
Fred violates the definition |

- If that node's height hasn't changed because of the insertion, then you are done. You may stop checking, and the insertion is complete.
- If the node's height has changed, but it does not violate the balance property, then you continue checking the next node in the path up to the root.
- If the node's height has changed and it now violates the balance property, then you need to perform one or two rotations to fix the problem, and then you are done.

Let's try some examples. Suppose I have the following AVL tree -- I now annotate the nodes with their heights:

If I insert *Ahmad*, take a look at the resulting tree:

The new node *Ahmad* has a height of zero, and when I travel the path up to
the root, I change *Baby Daisy's* height to one. However, her node is not
imbalanced, since the height of her subtrees are 0 and -1. Moving on, *Binky's*
height is unchanged, so we can stop -- the resulting tree is indeed an AVL tree.

However, suppose I now try to insert *Waluigi*. I get the following
tree:

Traveling from the new node to the root, I see that *Fred* violates the balance
condition. Its left child has a height of -1 and its right child has a height of 1.
I have to *rebalance* the tree.

And you insert a node into tree F, which changes
F's height from h-3 to h-2. Let's assume that F
is still an AVL tree, so we have this tree, where
A
C and
F are all AVL trees: |

You'll note that the tree is imbalanced, and it is imbalanced at node **B**. Let's
give some concrete examples:

You insert F into the tree:
Make sure you can match this example up with its general form above.
In this case, |
||

You insert "Jasper" into the tree:
Make sure you can match this example up with its general form above.
In this case, |
||

You insert an element into either E or G, changing its height,
but not violating the AVL property:
Make sure you can match this example up with its general form above. | ||

Let's take that last example as the general form of a Zig-Zig insertion:

In this case, the path from the imbalanced node (**B**) to the newly added node
starts with two right children. That's what makes it "Zig-Zig".
To rebalance the tree, you perform a rotation about
node **D**:

Node **D** is the new root of the subtree. Before the insertion, node **B**'s
height was *h-1*, so the height of the subtree has not changed because of the
rotation. Thus, after performing the rotation, you may return from your insertion:
you are left with a valid AVL tree.

If the path from the imbalanced node to the newly added node starts with two left children, then you have another Zig-Zig case (the mirror image). You treat it in the same way: rotate about the imbalanced node's left child.

Before going on, take a look at our example above where we inserted *Jasper*, which led
to the tree being imbalanced around *Daisy*. We can identify this as a
Zig-Zig case, so we can fix it by rotating about *Garth*:

Before |
After |

Here's another example, where we inserted *Waluigi* and that made the tree imbalanced
at *Fred*:

Before |
After |

To fix this, you perform two rotations. You rotate about node **D**, and then
you rotate about node **D** again. This is called a *double rotation* about
node **D**. Here are the two rotations:

Once again, the height of the subtree before deletion was *h-1*, so when you're
done with the double rotation, you are done -- your tree is balanced. Again, the
mirror image case is treated in the exact same manner.

Here's an example. Suppose our tree is the following, rather large tree:

And suppose we insert *Boo* into the tree:

Checking for balancing, we have to increment every height up to the root, and
the root node *Eunice* is imbalanced. Since the path to the new node starts
with a left child and a right child, this is a Zig-Zag case, and we need to perform
a double rotation about the grandchild of the imbalanced node -- *Daisy*.
Below is the result. We have a nicely balanced AVL tree!

- Its height is decremented by one.
An example of this is if
*Boo*is deleted from the above tree --*Calista*'s height is decremented by one. - Its height doesn't change and it stays balanced.
An example of this is if
*Fred*is deleted --*Luther's*height is unchanged and it is balanced. - Its height doesn't change, but it becomes imbalanced.
An example of this is if
*Dontonio*is deleted --*Eunice's*height is unchanged, but she is imbalanced.

- The parent's height is decremented by one. When this happens, the
parent will always still be a valid AVL tree. However, you need to now
check the parent's parent: you keep doing this until you return or you
reach the root of the tree. If Adam is deleted from the following AVL tree,
then this case applies. Ellen's height is decremented by one and Ellen is
still a valid AVL tree. However, Ellen's parent, Maureen, becomes unbalanced.
- The parent's height doesn't change and it stays balanced. When this happens you may return -- deletion is over.
- The parent's height doesn't change, but it becomes imbalanced. When this happens, you have to rebalance the subtree rooted at the parent. After rebalancing, the subtree's height may be one smaller than it was originally. If so, you must continue checking the parent's parent.

If you delete a node that has two children, then you will find the smallest child in the left subtree, delete it, and handle the rebalancing as though you had deleted that smallest child, rather than the original node. Once you have finished rebalancing the tree, you will replace the original node with the smallest child, just like you do with deletion in a BSTree.

To rebalance, you need to identify whether you are in a zig-zig situation or a zig-zag situation and rebalance accordingly. How do you do this? Look at the following picture:

Imbalanced Identification Picture |

The imbalanced node is **B**. If the height of subtree **C** is *h-3*, then
the height of **E** will be *h-2* and the tree is a Zig-Zig
-- you can rebalance by rotating about node **D**. If the
height of subtree **E** is *h-3*, then the height of **C** is *h-2* and
the tree is a Zig-Zag -- you rebalance
by doing a double rotation about the root of **C**. If both **C** and **E** have
heights of *h-2*, then you treat it as either a Zig-Zig or a Zig-Zag. Both work.
For the purposes of your lab, treat this case like a Zig-Zig.

The mirror image works the same way.

Let's look at some examples. First, suppose we delete *Calista* from the
following tree:

You're left with:

You check *Calista's* old parent -- *Binky* and although *Binky's*
height hasn't changed, the node is imbalanced. It's clearly a Zig-Zig tree, so
you rotate about *Baby Daisy* to yield the following tree:

Since *Baby Daisy* is the root, we're done.

Let's try a more complex example -- deleting *Eunice* from the following
tree:

First, *Eunice* has two children. So, we find the child with the greatest
key in *Eunice's* left subtree, which is *Dontonio*, delete it, and
replace *Eunice* with *Dontonio*. We start by deleting *Dontonio*:

And we start checking at *Eunice*. It is imbalanced. Looking at it, we see that
it's a Zig-Zag, so we have to double-rotate about *Fred*:

Now, the subtree rooted by *Fred* is balanced, but the subtree's height is one
less than it used to be, so we need to move to its parent and check it. Its
height is unchanged, and it is balanced, so we're done -- as a last step, we
replace *Eunice* with *Dontonio*:

Here's a larger example. We're going to delete

As in the previous example, *Daisy* has two children, so we find the node with
the largest key in *Daisy's* left subtree: *Calista*. We are going to
delete *Calista* and then replace *Daisy* with *Calista*. So, we
first delete *Calista*:

We start our checking with *Calista's* old parent: *Binky*. The node is
imbalanced, so we have to determine whether it is Zig-Zig or Zig-Zag. I'll redraw it
so it fits the Imbalanced Identification Picture:

In this example, *h* is three, and we're looking at the mirror image of the
Imbalanced Identification Picture.
You can see from the picture that the tree
is Zig-Zig, so we do a single rotation about *Baby Daisy*:

Since the tree rooted at *Baby Daisy* has a smaller height, we need to check
its parent (*Daisy*). *Daisy's* height is too high, so it needs to be
decremented, and since it's the root of the tree, we're done. We replace *Daisy*
with *Calista*, and return.

One final example. Let's delete

We're left with the following tree, and when we start checking *Frenchy's* old
parent, *Galois*, it is imbalanced:

I've drawn the subtrees for the Imbalanced Identification picture. *h* is two, and the
**C** and **E** subtrees are both of height *h-2*. Thus, we can treat it
either as a Zig-Zig or a Zig-Zag. We will treat it as a Zig-Zig (which you should do
in your lab) and do a single rotation about *Xavier*.

Since the subtree's height is the same as it was before, we can return.

To get that, you had to do a Zig-Zag balancing, and a Zig-Zig balancing.