Type tree O(n) O(n) O(log n) amortized O(log n) | Average Worst Case O(log n) O(log n) | |

Invented by Arne Andersson, Igal Galperin, Ronald L. Rivest |

In computer science, a **scapegoat tree** is a self-balancing binary search tree, invented by Arne Andersson and again by Igal Galperin and Ronald L. Rivest. It provides worst-case *O*(log *n*) lookup time, and *O*(log *n*) amortized insertion and deletion time.

## Contents

- Theory
- Insertion
- Sketch of proof for cost of insertion
- Deletion
- Sketch of proof for cost of deletion
- Lookup
- References

Unlike most other self-balancing binary search trees that provide worst case *O*(log *n*) lookup time, scapegoat trees have no additional per-node memory overhead compared to a regular binary search tree: a node stores only a key and two pointers to the child nodes. This makes scapegoat trees easier to implement and, due to data structure alignment, can reduce node overhead by up to one-third.

## Theory

A binary search tree is said to be weight-balanced if half the nodes are on the left of the root, and half on the right. An α-weight-balanced node is defined as meeting a relaxed weight balance criterion:

size(left) <= α*size(node) size(right) <= α*size(node)Where size can be defined recursively as:

function size(node) if node = nil return 0 else return size(node->left) + size(node->right) + 1 endAn α of 1 therefore would describe a linked list as balanced, whereas an α of 0.5 would only match almost complete binary trees.

A binary search tree that is α-weight-balanced must also be **α-height-balanced**, that is

_{1/α}(NodeCount) + 1

Scapegoat trees are not guaranteed to keep α-weight-balance at all times, but are always loosely α-height-balanced in that

height(scapegoat tree) <= log_{1/α}(NodeCount) + 1

This makes scapegoat trees similar to red-black trees in that they both have restrictions on their height. They differ greatly though in their implementations of determining where the rotations (or in the case of scapegoat trees, rebalances) take place. Whereas red-black trees store additional 'color' information in each node to determine the location, scapegoat trees find a **scapegoat** which isn't α-weight-balanced to perform the rebalance operation on. This is loosely similar to AVL trees, in that the actual rotations depend on 'balances' of nodes, but the means of determining the balance differs greatly. Since AVL trees check the balance value on every insertion/deletion, it is typically stored in each node; scapegoat trees are able to calculate it only as needed, which is only when a scapegoat needs to be found.

Unlike most other self-balancing search trees, scapegoat trees are entirely flexible as to their balancing. They support any α such that 0.5 < α < 1. A high α value results in fewer balances, making insertion quicker but lookups and deletions slower, and vice versa for a low α. Therefore in practical applications, an α can be chosen depending on how frequently these actions should be performed.

## Insertion

Insertion is implemented with the same basic ideas as an unbalanced binary search tree, however with a few significant changes.

When finding the insertion point, the depth of the new node must also be recorded. This is implemented via a simple counter that gets incremented during each iteration of the lookup, effectively counting the number of edges between the root and the inserted node. If this node violates the α-height-balance property (defined above), a rebalance is required.

To rebalance, an entire subtree rooted at a **scapegoat** undergoes a balancing operation. The scapegoat is defined as being an ancestor of the inserted node which isn't α-weight-balanced. There will always be at least one such ancestor. Rebalancing any of them will restore the α-height-balanced property.

One way of finding a scapegoat, is to climb from the new node back up to the root and select the first node that isn't α-weight-balanced.

Climbing back up to the root requires O(log *n*) storage space, usually allocated on the stack, or parent pointers. This can actually be avoided by pointing each child at its parent as you go down, and repairing on the walk back up.

To determine whether a potential node is a viable scapegoat, we need to check its α-weight-balanced property. To do this we can go back to the definition:

size(left) <= α*size(node) size(right) <= α*size(node)However a large optimisation can be made by realising that we already know two of the three sizes, leaving only the third having to be calculated.

Consider the following example to demonstrate this. Assuming that we're climbing back up to the root:

size(parent) = size(node) + size(sibling) + 1But as:

size(inserted node) = 1.The case is trivialized down to:

size[x+1] = size[x] + size(sibling) + 1Where x = this node, x + 1 = parent and size(sibling) is the only function call actually required.

Once the scapegoat is found, the subtree rooted at the scapegoat is completely rebuilt to be perfectly balanced. This can be done in O(*n*) time by traversing the nodes of the subtree to find their values in sorted order and recursively choosing the median as the root of the subtree.

As rebalance operations take O(*n*) time (dependent on the number of nodes of the subtree), insertion has a worst-case performance of O(*n*) time. However, because these worst-case scenarios are spread out, insertion takes O(log *n*) amortized time.

## Sketch of proof for cost of insertion

Define the Imbalance of a node *v* to be the absolute value of the difference in size between its left node and right node minus 1, or 0, whichever is greater. In other words:

Immediately after rebuilding a subtree rooted at *v*, I(*v*) = 0.

**Lemma:** Immediately before rebuilding the subtree rooted at *v*,

(

Proof of lemma:

Let

Since

## Deletion

Scapegoat trees are unusual in that deletion is easier than insertion. To enable deletion, scapegoat trees need to store an additional value with the tree data structure. This property, which we will call MaxNodeCount simply represents the highest achieved NodeCount. It is set to NodeCount whenever the entire tree is rebalanced, and after insertion is set to max(MaxNodeCount, NodeCount).

To perform a deletion, we simply remove the node as you would in a simple binary search tree, but if

NodeCount <= α*MaxNodeCountthen we rebalance the entire tree about the root, remembering to set MaxNodeCount to NodeCount.

This gives deletion its worst-case performance of O(n) time; however, it is amortized to O(log *n*) average time.

## Sketch of proof for cost of deletion

Suppose the scapegoat tree has

## Lookup

Lookup is not modified from a standard binary search tree, and has a worst-case time of O(log *n*). This is in contrast to splay trees which have a worst-case time of O(*n*). The reduced node memory overhead compared to other self-balancing binary search trees can further improve locality of reference and caching.