 Tree (data structure)

In computer science, a tree is a widelyused data structure that emulates a hierarchical tree structure with a set of linked nodes.
Mathematically, it is an ordered directed tree, more specifically an arborescence: an acyclic connected graph where each node has zero or more children nodes and at most one parent node. Furthermore, the children of each node have a specific order.
Contents
Terminology
A node is a structure which may contain a value, a condition, or represent a separate data structure (which could be a tree of its own). Each node in a tree has zero or more child nodes, which are below it in the tree (by convention, trees are drawn growing downwards). A node that has a child is called the child's parent node (or ancestor node, or superior). A node has at most one parent.
An internal node or inner node is any node of a tree that has child nodes. Similarly, an external node (also known as an outer node, leaf node, or terminal node), is any node that does not have child nodes.
The topmost node in a tree is called the root node. Being the topmost node, the root node will not have a parent. It is the node at which operations on the tree commonly begin (although some algorithms begin with the leaf nodes and work up ending at the root). All other nodes can be reached from it by following edges or links. (In the formal definition, each such path is also unique). In diagrams, it is typically drawn at the top. In some trees, such as heaps, the root node has special properties. Every node in a tree can be seen as the root node of the subtree rooted at that node. A free tree is a tree that is not rooted.
The height of a node is the length of the longest downward path to a leaf from that node. The height of the root is the height of the tree. The depth of a node is the length of the path to its root (i.e., its root path). This is commonly needed in the manipulation of the various self balancing trees, AVL Trees in particular. Conventionally, the value −1 corresponds to a subtree with no nodes, whereas zero corresponds to a subtree with one node.
A subtree of a tree T is a tree consisting of a node in T and all of its descendants in T. (This is different from the formal definition of subtree used in graph theory.^{[1]}) The subtree corresponding to the root node is the entire tree; the subtree corresponding to any other node is called a proper subtree (in analogy to the term proper subset).
Representations
There are many different ways to represent trees; common representations represent the nodes as dynamically allocated records with pointers to their children, their parents, or both, or as items in an array, with relationships between them determined by their positions in the array (e.g., binary heap).
Trees and graphs
The tree data structure can be generalized to represent directed graphs by removing the constraints that a node may have at most one parent, and that no cycles are allowed. Edges are still abstractly considered as pairs of nodes, however, the terms parent and child are usually replaced by different terminology (for example, source and target). Different implementation strategies exist, for example adjacency lists.
Relationship with trees in graph theory
In graph theory, a tree is a connected acyclic graph; unless stated otherwise, trees and graphs are undirected. There is no onetoone correspondence between such trees and trees as data structure. We can take an arbitrary undirected tree, arbitrarily pick one of its vertices as the root, make all its edges directed by making them point away from the root node  producing an arborescence  and assign an order to all the nodes. The result corresponds to a tree data structure. Picking a different root or different ordering produces a different one.
Traversal methods
Main article: Tree traversalStepping through the items of a tree, by means of the connections between parents and children, is called walking the tree, and the action is a walk of the tree. Often, an operation might be performed when a pointer arrives at a particular node. A walk in which each parent node is traversed before its children is called a preorder walk; a walk in which the children are traversed before their respective parents are traversed is called a postorder walk; a walk in which a node's left subtree, then the node itself, and then finally its right subtree are traversed is called an inorder traversal. (This last scenario, referring to exactly two subtrees, a left subtree and a right subtree, assumes specifically a binary tree.) Here inorder is like infix expression,postorder is like postfix expression and preorder is like prefix expression.
Common operations
 Enumerating all the items
 Enumerating a section of a tree
 Searching for an item
 Adding a new item at a certain position on the tree
 Deleting an item
 Removing a whole section of a tree (called pruning)
 Adding a whole section to a tree (called grafting)
 Finding the root for any node
Common uses
 Manipulate hierarchical data
 Make information easy to search (see tree traversal and binary search tree)
 Manipulate sorted lists of data
 As a workflow for compositing digital images for visual effects
 Router algorithms
See also
Other trees
References
 ^ Eric W. Weisstein "Subtree." From MathWorld—A Wolfram Web Resource. http://mathworld.wolfram.com/Subtree.html
 Notes
 Donald Knuth. The Art of Computer Programming: Fundamental Algorithms, Third Edition. AddisonWesley, 1997. ISBN 0201896834 . Section 2.3: Trees, pp. 308–423.
 Thomas H. Cormen, Charles E. Leiserson, Ronald L. Rivest, and Clifford Stein. Introduction to Algorithms, Second Edition. MIT Press and McGrawHill, 2001. ISBN 0262032937 . Section 10.4: Representing rooted trees, pp. 214–217. Chapters 12–14 (Binary Search Trees, RedBlack Trees, Augmenting Data Structures), pp. 253–320.
External links
 Description from the Dictionary of Algorithms and Data Structures
 STLlike C++ tree class
 Description of tree data structures from ideainfo.8m.com
 flash actionscript 3 opensource implementation of tree and binary tree — opensource library
 WormWeb.org: Interactive Visualization of the C. elegans Cell Tree  Visualize the entire cell lineage tree of the nematode C. elegans (javascript)
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