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262 lines
No EOL
14 KiB
Markdown
---
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title: On the rules of the red-black tree
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description: |
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Shower thoughts on the rules of the red-black tree.
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tags:
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- red-black trees
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- balanced trees
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---
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## Introduction
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Have you ever thought about the red-black tree rules in more depth? Why are they
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formulated the way they are? How come they keep the tree balanced? Let's go through
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each of the red-black tree rules and try to change, break and contemplate about
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them.
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We expect that you are familiar with the following set of the rules[^1]:
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1. Every node is either red or black.
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2. The root is black.
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3. Every leaf (`nil`) is black.
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4. If a node is red, then both its children are black.
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5. For each node, all simple paths from the node to descendant leaves contain the
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same number of black nodes.
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Each section will go into _reasonable_ details of each rule.
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## 1ª Every node is either red or black.
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OK… This one is very simple. It is just a definition and is used in all other
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rules. Not much to talk about here. Or is there?
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### Do I really need the nodes to be explicitly colored?
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The answer is no. Balancing of the red-black trees is “enforced” by the 4th and
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5th rule in the enumeration above. There are many ways you can avoid using colors.
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#### Black height
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We mentioned the 4th and 5th rule and that it enforces the balancing. What does
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it mean for us?
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Well, we definitely do not have to use the colors, which even as a _boolean_ flag
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would take at least 1 byte of space (and usually even more), cause… well, it is
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easier for the CPU to work with words rather than single bits.
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We could use the black height, couldn't we? It would mean more memory used, cause
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it should be ideally big and unsigned. Can we tell the color of a node from the
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black height? Of course we can, if my child has the same black height as I do,
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it means that there was no black node added on the path between us and therefore
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my child would be colored red.
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Example of a red-black tree that keeps count of black nodes on paths to the
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leaves follows:
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We mark the _black heights_ in superscript. You can see that all leaves have the
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black height equal to $1$. Let's take a look at some of the interesting cases:
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* If we take a look at the node with $\text{key} = 9$, we can see that it is
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coloured red and its black height is 1, because it is a leaf.
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Let's look at its parent (node with $\text{key} = 8$). On its left side it has
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`nil` and on its right side the $9$. And its black height is still $1$, cause
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except for the `nil` leaves, there are no other black nodes.
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We can clearly see that if a node has the same black height as its parent, it
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is a red node.
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* Now let's take a look at the root with $\text{key} = 3$. It has a black height
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of 3. Both of its children are black nodes and have black height of 2.
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We can see that if a node has its height 1 lower than its parent, it is a black
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node.
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The reasoning behind it is rather simple, we count the black nodes all the way
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to the leaves, therefore if my parent has a higher black height, it means that
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on the path from me to my parent there is a black node, but the only node added
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is me, therefore I must be black.
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#### Isomorphic trees
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One of the other ways to avoid using color is storing the red-black tree in some
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isomorphic tree. The structure of 2-3-4 tree allows us to avoid using the color
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completely. This is a bit different approach, cause we would be basically using
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different tree, so we keep this note in just as a “hack”.
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## 2ª The root is black.
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This rule might seem like a very important one, but overall is not. You can safely
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omit this rule, but you also need to deal with the consequences.
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Let's refresh our memory with the algorithm of _insert fixup_:
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```
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WHILE z.p.color == Red
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IF z.p == z.p.p.left
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y = z.p.p.right
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IF y.color == Red
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z.p.color = Black
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y.color = Black
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z.p.p.color = Red
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z = z.p.p
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ELSE
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IF z == z.p.right
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z = z.p
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Left-Rotate(T, z)
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z.p.color = Black
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z.p.p.color = Red
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Right-Rotate(T, z.p.p)
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ELSE (same as above with “right” and “left” exchanged)
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T.root.color = Black
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```
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:::tip
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If you have tried to implement any of the more complex data structures, such as
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red-black trees, etc., in a statically typed language that also checks you for
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`NULL`-correctness (e.g. _mypy_ or even C# with nullable reference types), you
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might have run into numerous issues in the cases where you are 100% sure that you
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cannot obtain `NULL` because of the invariants, but the static type checking
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doesn't know that.
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The issue we hit with the _insert fixup_ is very similar.
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:::
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You might not realize the issue at the first sight, but the algorithm described
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with the pseudocode above expects that the root of the red-black tree is black by
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both relying on the invariant in the algorithm and afterwards by enforcing the
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black root property.
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If we decide to omit this condition, we need to address it in the pseudocodes
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accordingly.
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| Usual algorithm with black root | Allowing red root |
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| :-----------------------------: | :---------------: |
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## 3ª Every leaf (`nil`) is black.
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Now, this rule is a funny one. What does this imply and can I interpret this in
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some other way? Let's go through some of the possible ways I can look at this and
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how would they affect the other rules and balancing.
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We will experiment with the following tree:
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We should start by counting the black nodes from root to the `nil` leaves based
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on the rules. We have multiple similar paths, so we will pick only the interesting
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ones.
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1. What happens if we do not count the `nil` leaves?
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2. What happens if we consider leaves the nodes with _no descendants_, i.e. both
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of node's children are `nil`?
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3. What happens if we do not count the `nil` leaves, but consider nodes with at
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least one `nil` descendant as leaves?
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| path | black nodes | 1ª idea | 2ª idea | 3ª idea |
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| ------------------------: | ----------: | ------: | ------: | ------: |
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| `3 → 1 → 0 → nil` | 4 | 3 | 4 | 3 |
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| `3 → 5 → 7 → 8 → nil` | 4 | 3 | - | 3 |
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| `3 → 5 → 7 → 8 → 9 → nil` | 4 | 3 | 4 | 3 |
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First idea is very easy to execute and it is also very easy to argue about its
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correctness. It is correct, because we just subtract one from each of the paths.
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This affects **all** paths and therefore results in global decrease by one.
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Second idea is a bit more complicated. We count the `nil`s, so the count is $4$
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as it should be. However, there is one difference. Second path no longer satisfies
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the condition of a _leaf_. Technically it relaxes the 5th rule, because we leave
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out some of the nodes. We should probably avoid that.
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:::caution
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With the second idea, you may also feel that we are “bending” the rules a bit,
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especially the definition of the “leaf” nodes.
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Given the definition of the red-black tree, where `nil` is considered to be an
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external node, we have decided that bending it a bit just to stir a thought about
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it won't hurt anybody. :wink:
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:::
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## 4ª If a node is red, then both its children are black.
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This rule might seem rather silly on the first look, but there are 2 important
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functions:
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1. it allows the algorithms to _“notice”_ that something went wrong (i.e. the
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tree needs to be rebalanced), and
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2. it holds the balancing and height of the tree _“in check”_ (with the help of
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the 5th rule).
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When we have a look at the algorithms that are used for fixing up the red-black
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tree after an insertion or deletion, we will notice that all the algorithms need
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is the color of the node.
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> How come it is the only thing that we need?
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> How come such naïve thing can be enough?
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Let's say we perform an insertion into the tree… We go with the usual and pretty
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primitive insertion into the binary-search tree and then, if needed, we “fix up”
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broken invariants. _How can that be enough?_ With each insertion and deletion we
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maintain the invariants, therefore if we break them with one operation, there's
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only one path on which the invariants were _felled_. If we know that rest of the
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tree is correct, it allows us to fix the issues just by propagating it to the
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root and _abusing_ the siblings (which are, of course, correct red-black
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subtrees) to fix or at least partially mitigate the issues and propagate them
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further.
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Let's assume that we do not enforce this rule, you can see how it breaks the
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balancing of the tree below.
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| Enforcing this rule | Omitting this rule |
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| :-----------------: | :----------------: |
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We can create a **big** subtree with only red nodes and **even** when keeping
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the rest of the rules maintained, it will break the time complexity. It stops us
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from “hacking” the black height requirement laid by the 5th rule.
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## 5ª For each node, all simple paths from the node to descendant leaves contain the same number of black nodes.
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As it was mentioned, with the 4th rule they hold the balancing of the red-black
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tree.
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:::tip
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An important observation here is the fact that the red-black tree is a
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**height**-balanced tree.
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:::
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Enforcing this rule (together with the 4th rule) keeps the tree balanced:
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1. 4th rule makes sure we can't “hack” this requirement.
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2. This rule ensures that we have “similar”[^2] length to each of the leaves.
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:::tip AVL tree
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You might have heard about an _AVL tree_ before. It is the first self-balanced
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tree to be ever introduced and works in a very similar nature as the red-black
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tree, the only difference is that it does not deal with the _black height_, but
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the height in general.
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If you were to compare AVL with the red-black tree, you can say that AVL is much
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more strict while red-black tree can still maintain the same asymptotic time
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complexity for the operations, but having more relaxed rules.
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:::
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[^1]: CORMEN, Thomas. Introduction to algorithms. Cambridge, Mass: MIT Press, 2009. isbn 9780262033848.
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[^2]: red nodes still exist |