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# Balanced BSTs

Balanced Binary Search Trees are an even more specific subcategory of binary trees that have an important property: they are always bushy.

## B Trees (2-4 Trees)

The basic idea: Nodes can hold multiple values now! When nodes have too many values, we will split it.
A 2-4 tree is named such because each parent can have 2 to 4 children. Another constraint we will put on is a limit on the number of items allowed in a single node, so that we can guarantee that searching a single node will always be
$\Theta(n).$

### Adding Values to a B-Tree

Adding values to a B Tree can be a bit tricky because we need to make sure all the properties are still followed. Here are some example scenarios:
If a node already has 2 or more children, place the new value in one of its existing children. If a node is full (reaches the limit), we must split the node by moving one value up to the parent and creating another child node. Here, we'll use a limit of 3. ### Properties of B Trees

• Searching in a single node is constant runtime since the limit is a constant.
• All leaves must be the same distance from the root.
• A non-leaf node with k items must have k+1 children.
• The height of a B tree is guaranteed to be
$\Theta(\log(n))$
because it is bushy.

## Red-Black Trees and Tree Rotation

The basic idea: Let's try to represent B trees in a binary tree format. That means that every parent can only have 2 children! In order to do this, we'll add an extra color property to each node.
Black nodes are just like any normal binary tree node, but Red nodes represent the nodes in B Trees that have more than one value. For example, let's convert the B Tree we were working with before into a RB Tree. In order to make our lives easier, we'll restrict our Red Black trees into left leaning red black trees which can only have red nodes on the left.

### Tree Rotation

In order to ensure that adding new nodes won't break the Red Black Tree structure, we will use a concept called tree rotation which swaps around nodes. There are two rotations, a left rotation and a right rotation, which move a child node up to replace its parent. For example, a left rotation moves the right node up and left to replace the parent.
A "left rotation on 7" looks like this: Notice that the 8 gets moved to be a right child of 7 after the rotation! This is necessary to preserve the binary tree structure.
A "right rotation on 7" looks like this: Here, the 6 gets moved to be a left child of 7.
If you want to see how these rotations can be implemented into the insert algorithm, try the homework on implementing a LLRB Tree! Below is a brief outline on how insert works:
• Always add values to a leaf node as a red node first. Follow normal sorted binary tree rules.
• If the link is leaning right, rotate the tree to make it left leaning.
• If a node already has a red link to the left, temporarily add it to the right also as a red link.
• Then, flip the color of all links connected to the node (if previously black, turn red; if previously red, turn black)
• Might need to fix right-leaning red nodes that are created as a result
• If a node has red links to both parent and child, rotate it such that it becomes the above case, and then handle that case like you did before.

### Properties of Red Black Trees

Like B Trees, Red Black Trees have some important properties that allow them to be easily distinguishable.
• Red Black trees have a one-to-one correspondence with B trees. That means for every Red Black tree, there is exactly one B Tree that represents the same connections. This also means that a Red Black Tree will have the same runtimes as their corresponding B Trees. (Take a linear algebra course to learn more about isomorphisms 🙂 )
• Every node must have the same number of black nodes in between itself and the root. This might be a bit surprising at first, but remember that their corresponding B Tree is always bushy, and red links mean a multi-value node in a B Tree.