Binary search tree

In computer science, a binary search tree (BST), also called an ordered or sorted binary tree, is a rooted binary tree data structure whose internal nodes each store a key greater than all the keys in the node’s left subtree and less than those in its right subtree. A binary tree is a type of data structure for storing data such as numbers in an organized way. Binary search trees allow binary search for fast lookup, addition and removal of data items, and can be used to implement dynamic sets and lookup tables. The order of nodes in a BST means that each comparison skips about half of the remaining tree, so the whole lookup takes time proportional to the binary logarithm of the number of items stored in the tree. This is much better than the linear time required to find items by key in an (unsorted) array, but slower than the corresponding operations on hash tables. Several variants of the binary search tree have been studied.

Binary search tree

Type

tree

Invented

1960

Invented by

P.F. Windley, A.D. Booth, A.J.T. Colin, and T.N. Hibbard

Time complexity in big O notation

Algorithm	Average	Worst case
Space	$O(n)$	$O(n)$
Search	$O(log n)$	$O(n)$
Insert	$O(log n)$	$O(n)$
Delete	$O(log n)$	$O(n)$

A binary search tree of size 9 and depth 3, with 8 at the root. The leaves are not drawn.

DefinitionEdit

A binary search tree is a rooted binary tree, whose internal nodes each store a key (and optionally, an associated value), and each has two distinguished sub-trees, commonly denoted left and right. The tree additionally satisfies the binary search property: the key in each node is greater than or equal to any key stored in the left sub-tree, and less than or equal to any key stored in the right sub-tree.^[1]^: 287 The leaves (final nodes) of the tree contain no key and have no structure to distinguish them from one another. The shape of the binary search tree depends entirely on the order of insertions and deletions and can become degenerate.

Often, the information represented by each node is a record rather than a single data element. However, for sequencing purposes, nodes are compared according to their keys rather than any part of their associated records. The major advantage of binary search trees over other data structures is that the related sorting algorithms and search algorithms such as inorder traversal can be very efficient.^{[note 1]}

Binary search trees are a fundamental data structure used to construct more abstract data structures such as sets, multisets, and associative arrays. There has been a lot of research to prevent degeneration of the tree resulting in worst case time complexity of $O(n)$ (for details see section Types).

Order relationEdit

Binary search requires an order relation by which every element (item) can be compared with every other element in the sense of a total preorder. The part of the element which effectively takes place in the comparison is called its key. Whether duplicates, i. e. different elements with the same key, shall be allowed in the tree or not, does not depend on the order relation, but on the underlying set, in other words: on the application only. For a search function supporting and handling duplicates in a tree, see section Searching with duplicates allowed.

In the context of binary search trees, a total preorder is realized most flexibly by means of a three-way comparison subroutine.

OperationsEdit

Binary search trees support three main operations: lookup (checking whether a key is present), insertion, and deletion of an element. The latter two possibly change the structural arrangement of the nodes in the tree, whereas the first one is a navigating and read-only operation. Other read-only operations are traversal, verification, etc.

SearchingEdit

Searching in a binary search tree for a specific key can be programmed recursively or iteratively.

We begin by examining the root node. If the tree is ${\text{nil}}$ , the key we are searching for does not exist in the tree. Otherwise, if the key equals that of the root, the search is successful and we return the node. If the key is less than that of the root, we search the left subtree. Similarly, if the key is greater than that of the root, we search the right subtree. This process is repeated until the key is found or the remaining subtree is ${\text{nil}}$ . If the searched key is not found after a ${\text{nil}}$ subtree is reached, then the key is not present in the tree.

Recursive searchEdit

The following pseudocode implements the BST search procedure through recursion.^[1]^: 290

 Tree-Search(x, key)
   if x = NIL or key = x.key then
     return x
   if key < x.key then
     return Tree-Search(x.left, key)
   else
     return Tree-Search(x.right, key)
   end if

Iterative searchEdit

The recursive version of the search can be "unrolled" into a while loop. On most machines, the iterative version is found to be more efficient.^[1]^: 291

 Iterative-Tree-Search(x, key)
   while x ≠ NIL and key ≠ x.key then
     if key < x.key then
       x := x.left
     else
       x := x.right
     end if
   repeat
   return x

Since the search may proceed till some leaf node, the running time complexity of BST search is $O(h)$ where $h$ is the height of the tree. However, the worst case for BST search is $O(n)$ where $n$ is the total number of nodes in the BST, because an unbalanced BST may degenerate to a linked list. However, if the BST is height-balanced the height is $O(\log n)$ .^[1]^: 290

Maximum and minimumEdit

Operations such as finding a node in a BST whose key is the maximum or minimum are critical in certain operations, such as determining the successor and predecessor of nodes. Following is the pseudocode for the operations.^[1]^{: 291–292}

 Tree-Maximum(x)
   while x.right ≠ NIL then
     x := x.right
   repeat
   return x

 Tree-Minimum(x)
   while x.left ≠ NIL then
     x := x.left
   repeat
   return x

Successor and predecessorEdit

For certain operations, given a node $x$ , we need to find the successor or predecessor of $x$ . Assuming all the keys of the BST are distinct, the successor of a node $x$ in BST is the node with the smallest key greater than $x$ 's key. On the other hand, the predecessor of a node $x$ in BST is the node with the largest key smaller than $x$ 's key. Following is pseudocode for finding the successor and predecessor of a node $x$ in BST.^[1]^{: 292–293}

 Tree-Successor(x)
   if x.right ≠ NIL then
     return Tree-Minimum(x.right)
   end if
   y := x.parent
   while y ≠ NIL and x = y.right then
     x := y
     y := y.parent
   repeat
   return y

 Tree-Predecessor(x)
   if x.left ≠ NIL then
     return Tree-Maximum(x.left)
   end if
   y := x.parent
   while y ≠ NIL and x = y.left then
     x := y
     y := y.parent
   repeat
   return y

TraversalEdit

A BST can be traversed through three basic algorithms: inorder, preorder, and postorder tree walk.^[1]^: 287

Inorder tree walk: Nodes from the left subtree get visited first, followed by the root node and right subtree.
Preorder tree walk: The root node gets visited first, followed by left and right subtrees.
Postorder tree walk: Nodes from the left subtree get visited first, followed by the right subtree, and finally the root.

Following is a recursive implementation of the tree walks.^[1]^{: 287–289}

 Inorder-Tree-Walk(x)
   if ≠ NIL then
     Inorder-Tree-Walk(x.left)
     visit node
     Inorder-Tree-Walk(x.right)
   end if

 Preorder-Tree-Walk(x)
   if ≠ NIL then
     visit node
     Preorder-Tree-Walk(x.left)
     Preorder-Tree-Walk(x.right)
   end if

 Postorder-Tree-Walk(x)
   if ≠ NIL then
     Postorder-Tree-Walk(x.left)
     Postorder-Tree-Walk(x.right)
     visit node
   end if

HeightEdit

Height of the binary search tree is defined as the maximum of the heights of left subtree and right subtree incremented by a factor of 1. Following is a recursive procedure for calculating the height of the BST given a root $x$ :^[2]^: 303-304

 Tree-Height(x)
   if x = NIL then
     return -1
   end if
   left_height := Tree-Height(x.left)
   right_height := Tree-Height(x.right)
   if left_height > right_height then
     return left_height + 1
   else
     return right_height + 1
   end if

InsertionEdit

Operations such as insertion and deletion cause the BST representation to change dynamically. The data structure must be modified in such a way that the properties of BST continue to hold. New nodes are inserted as leaf nodes in the BST.^[1]^{: 294–295} Following is an iterative implementation of the insertion operation.^[1]^: 294

1    Tree-Insert(T, z)
2      y := NIL
3      x := T.root
4      while x ≠ NIL do
5        y := x
6        if z.key < x.key then
7          x := x.left
8        else
9          x := x.right
10       end if
11    repeat
12    z.parent := y
13    if y = NIL then
14      T.root := z
15    else if z.key < y.key then
16      y.left := z
17    else
18      y.right := z
19    end if

The procedure maintains a "trailing pointer" y as a parent of x. After initialization on line 2, the while loop along the lines 4-11 causes the pointers to be updated. If y is nil, the BST is empty, thus z is inserted as the root node of the binary search tree T, if it isn't nil, we compare the keys on the lines 15-19 and insert the node accordingly.^[1]^: 295

DeletionEdit

Fig. 2: Binary search tree special cases deletion illustration.

Deletion of a node ${\text{z}}$ from a binary search tree ${\text{T}}$ has three cases:^[1]^: 295

If ${\text{z}}$ is a leaf node, we remove ${\text{z}}$ by replacing its parent with ${\text{nil}}$ as its child.
If ${\text{z}}$ has only one child, we elevate that child—either left or right—to ${\text{z}}$ 's position by modifying ${\text{z}}$ 's parent by replacing it with ${\text{z}}$ 's child, as shown in fig. 2 part (a) and (b).
If ${\text{z}}$ ${\text{z}}$ has both a left and right child, we find ${\text{z}}$ ${\text{z}}$ 's successor ${\text{y}}$ ${\text{y}}$ and have it take ${\text{z}}$ ${\text{z}}$ 's position in the tree. ${\text{z}}$ ${\text{z}}$ 's original right subtree becomes ${\text{y}}$ ${\text{y}}$ 's new right subtree and ${\text{z}}$ ${\text{z}}$ 's left subtree becomes ${\text{y}}$ ${\text{y}}$ 's new left subtree respectively. However, this case isn't trivial, since it depends on the position of ${\text{y}}$ ${\text{y}}$ in the BST.^[1]^: 296
1. If ${\text{y}}$ is ${\text{z}}$ 's right child, we elevate ${\text{y}}$ by leaving ${\text{y}}$ 's right child alone, as shown in fig. 2 part (c).
2. If ${\text{y}}$ isn't the right child, but lies within the right subtree and have a left child—either ${\text{nil}}$ or a subtree—we first replace ${\text{y}}$ by its own right child, and then replace ${\text{z}}$ with ${\text{y}}$ , as shown in fig. 2 part (d).

Following is a pseudocode for the deletion operation in a binary search tree.^[1]^: 296-298

1    Tree-Delete(T, z)
2      if z.left = NIL then
3        Subtree-Shift(T, z, z.right)
4      else if z.right = NIL then
5        Subtree-Shift(T, z, z.left)
6      else
7        y := Tree-Successor(z)
8        if y.parent ≠ z then
9          Subtree-Shift(T, y, y.right)
10         y.right := z.right
11         y.right.parent := y
12       end if
13       Subtree-Shift(T, z, y)
14       y.left := z.left
15       y.left.parent := y
16     end if

1    Subtree-Shift(T, u, v)
2      if u.parent = NIL then
3        T.root := v
4      else if u = u.parent.left then
5        u.parent.left := v
5      else
6        u.parent.right := v
7      end if
8      if v ≠ NIL then
9        v.parent := u.parent
10     end if

The ${\text{Tree-Delete}}$ procedure deals with the 3 special cases mentioned above. Lines 2-3 deal with case 1; lines 4-5 deal with case 2 and lines 6-16 for case 3 respectively. The helper function ${\text{Tree-Shift}}$ is used within the deletion algorithm for the purpose of replacing the node ${\text{u}}$ with ${\text{v}}$ in the binary search tree ${\text{T}}$ .^[1]^: 298

Examples of applicationsEdit

SortEdit

A binary search tree can be used in sorting algorithm implementation. The process involves inserting all the elements which are to be sorted and performing inorder traversal. This method is similar to that of quicksort where each node corresponds to a partitioning item that subdivides its descendants into smaller keys and larger keys.^[3]

Priority queue operationsEdit

Binary search trees are used in implementing priority queues, using the element or node's key as priorities. Adding new elements to the queue follows the regular BST ${\text{Tree-Insert}}$ operation; but the removal operation depends on the type of priority queue:^[4]

If it's an ascending order priority queue, removal of an element with the lowest priority is done through leftward traversal of the BST i.e. ${\text{Tree-Minimum }}$ .
On the other hand, if it's a descending order priority queue, removal of an element with the highest priority is done through rightward traversal of the BST i.e. ${\text{Tree-Maximum }}$ .

TypesEdit

There are many types of binary search trees. AVL trees and red–black trees are both forms of self-balancing binary search trees. A splay tree is a binary search tree that automatically moves frequently accessed elements nearer to the root. In a treap (tree heap), each node also holds a (randomly chosen) priority and the parent node has higher priority than its children.

Tango trees is an online binary search tree which is optimized for fast searches and achieves an $O(\log \log n)$ competitive ratio relative to the offline optimal binary search tree such as self-balancing binary search trees, while only using $O(\log \log n)$ additional bits of memory per node.^[5]

T-tree is a balanced binary search tree optimized to reduce storage space overhead which are used for in-memory databases.^[6]

A degenerate tree is a binary search tree which contains $n$ nodes and has height of $n-1$ . The performance or time complexity of a lookup operation is essentially identical with that of a linear search i.e. $O(n)$ , which is alike that of data structures like arrays or linked lists.^[7]

Performance comparisonsEdit

In regards to performance characteristics of binary search trees, a study shows that Treaps perform better on average case, while red–black tree was found to have the smallest number of performance variations.^[8]

Optimal binary search treesEdit

Optimal binary search tree is a theoretical computer science problem which deals with constructing an "optimal" binary search trees that enables smallest possible search time for a given sequence of accesses.^[9]^{: 449–450} The computational cost required to maintain an "optimal" search tree can be justified if search is more dominant activity in the tree than insertion or deletion.^[10]^[9]^: 449

Threaded binary treesEdit

A threaded binary search tree is an accessorial version of a binary tree whose $nil$ pointers—either left or right fields of a node—points to the inorder successor or inorder predecessor of the given nodes such that efficient utilization of the placeholders fields are performed.^[2]^: 311-312 Threading is classified into two categories:^[2]^: 312

One-way threading: The left or right pointer field of the nodes, holds a reference to the inorder predecessor or inorder successor, but not both.
Two-way threading: The left and right pointer fields hold the references to the inorder predecessor and inorder successor respectively.

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Metasyntactic variable, which is released under the
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Binary search tree

DefinitionEdit

Order relationEdit

OperationsEdit

SearchingEdit

Recursive searchEdit

Iterative searchEdit

Maximum and minimumEdit

Successor and predecessorEdit

TraversalEdit

HeightEdit

InsertionEdit

DeletionEdit

Examples of applicationsEdit

SortEdit

Priority queue operationsEdit

TypesEdit

Performance comparisonsEdit

Optimal binary search treesEdit

Threaded binary treesEdit

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