In graph theory, an m-ary tree (for nonnegative integers m) (also known as n-ary, k-ary or k-way tree) is an arborescence (or, for some authors, an ordered tree) in which each node has no more than m children. A binary tree is the special case where m = 2, and a ternary tree is another case with m = 3 that limits its children to three.
Types of m-ary trees
A full m-ary tree is an m-ary tree where within each level every node has 0 or m children. A complete m-ary tree (or, less commonly, a perfect m-ary tree) is a full m-ary tree in which all leaf nodes are at the same depth.
Properties of m-ary trees
For an m-ary tree with height h, the upper bound for the maximum number of leaves is . The height h of an m-ary tree does not include the root node, with a tree containing only a root node having a height of 0. The height of a tree is equal to the maximum depth D of any node in the tree. The total number of nodes in a perfect m-ary tree is , while the height h is By the definition of Big-Ω, the maximum depth
The height of a complete m-ary tree with n nodes is . The total number of possible m-ary tree with n nodes is (which is a Catalan number).
Traversal methods for m-ary trees
Traversing a m-ary tree is very similar to traversing a binary tree. The pre-order traversal goes to parent, left subtree and the right subtree, and for traversing post-order it goes by left subtree, right subtree, and parent node. For traversing in-order, since there are more than two children per node for m > 2, one must define the notion of left and right subtrees. One common method to establish left/right subtrees is to divide the list of children nodes into two groups. By defining an order on the m children of a node, the first nodes would constitute the left subtree and nodes would constitute the right subtree.
Convert a m-ary tree to binary tree
Using an array for representing a m-ary tree is inefficient, because most of the nodes in practical applications contain less than m children. As a result, this fact leads to a sparse array with large unused space in the memory. Converting an arbitrary m-ary tree to a binary tree would only increase the height of the tree by a constant factor and would not affect the overall worst-case time complexity. In other words, since .
First, we link all the immediate children nodes of a given parent node together in order to form a link list. Then, we keep the link from the parent to the first (i.e., the leftmost) child and remove all the other links to the rest of the children. We repeat this process for all the children (if they have any children) until we have processed all the internal nodes and rotate the tree by 45 degrees clockwise. The tree obtained is the desired binary tree obtained from the given m-ary tree.
Methods for storing m-ary trees
Arrays
m-ary trees can also be stored in breadth-first order as an implicit data structure in arrays, and if the tree is a complete m-ary tree, this method wastes no space. In this compact arrangement, if a node has an index i, its c-th child in range { 1,…,m} is found at index , while its parent (if any) is found at index (assuming the root has index zero, meaning a 0-based array). This method benefits from more compact storage and better locality of reference, particularly during a preorder traversal. The space complexity of this method is .
Pointer-based
Each node would have an internal array for storing pointers to each of its children:
Compared to array-based implementation, this implementation method has superior space complexity of .
Enumeration of m-ary trees
Listing all possible m-ary trees is useful in many disciplines as a way of checking hypotheses or theories. Proper representation of m-ary tree objects can greatly simplify the generation process. One can construct a bit sequence representation using the depth-first search of a m-ary tree with n nodes indicating the presence of a node at a given index using binary values. For example, the bit sequence x=1110000100010001000 is representing a 3-ary tree with n=6 nodes as shown below.