Tree Data Structure Java Library

Description

This Library contains different implementations of the tree data structures, such as K-ary, binary, expression trees etc.

Requirements

The Library requires Java SE Development Kit 7 or higher

Gradle dependency

dependencies {
	compile 'com.scalified:tree:0.2.5'
}

Theory

Definition

A tree data structure can be defined recursively (locally) as a collection of nodes (starting at a root node), where each node is a data structure consisting of a value, together with a list of references to nodes (the children), with the constraints that no reference is duplicated, and none points to the root

A tree is a (possibly non-linear) data structure made up of nodes or vertices and edges without having any cycle. The tree with no nodes is called the null or empty tree. A tree that is not empty consists of a root node and potentially many levels of additional nodes that form a hierarchy

Terminology

Node - a single point of a tree
Edge - line, which connects two distinct nodes
Root - top node of the tree, which has no parent
Parent - a node, other than the root, which is connected to other successor nodes
Child - a node, other than the root, which is connected to predecessor
Leaf - a node without children
Path - a sequence of nodes and edges connecting a node with a descendant
Path Length - number of nodes in the path - 1
Ancestor - the top parent node of the path
Descendant - the bottom child node of the path
Siblings - nodes, which have the same parent
Subtree - a node in a tree with all of its proper descendants, if any
Node Height - the number of edges on the longest downward path between that node and a leaf
Tree Height - the number of edges on the longest downward path between the root and a leaf (root height)
Depth (Level) - the path length between the root and the current node
Ordered Tree - tree in which nodes has the children ordered
Labeled Tree - tree in which a label or value is associated with each node of the tree
Expression Tree - tree which specifies the association of an expression's operands and its operators in a uniform way, regardless of whether the association is required by the placement of parentheses in the expression or by the precedence and associativity rules for the operators involved
Branching Factor - maximum number of children a node can have
Pre order - a form of tree traversal, where the action is called firstly on the current node, and then the pre order function is called again recursively on each of the subtree from left to right
Post order - a form of tree traversal, where the post order function is called recursively on each subtree from left to right and then the action is called

Trees Representation

Array-of-Pointers

One of the simplest ways of representing a tree is to use for each node a structure, which contains an array of pointers to the children of that node
The maximum number of children a node can have is known as branching factor (BF)
The ith component of the array at a node contains a pointer to the ith child of that node. A missing child can be represented by a null pointer

Array-of-Pointers representation makes it possible to quickly access the ith child of any node
This representation, however, is very wasteful of space when only a few nodes in the tree have many children. In this case, most of the pointers in the arrays will be null

The Library, however, is optimized for using Array-of-Pointers representation likewise it is done in the java.util.ArrayList. The subtrees size grows automatically in case of reaching the limit. Additionally you may specify what is called a branching factor, that the initial number of subtrees before grow (like capacity in the java.util.ArrayList)

Leftmost-Child—Right-Sibling

In the Leftmost-Child—Right-Sibling representation each node contains a pointer to its leftmost child; a node does not have pointers to any of its other children. In order to locate the second and subsequent children of a node n, the children link list created, in which each child c points to the child of n immediately to the right of c. That node is called the right sibling of c

In the figure above n3 is the right sibling of n2, n4 is the right sibling of n3, and n4 has no right sibling. The children of n1 can be found by following its leftmost-child pointer to n2, then the right-sibling pointer to n3, and then the right-sibling pointer of n3 to n4. n4, in turn has no right-sibling pointer (it is null indeed), therefore n1 has no more children
The figure above can be represented in such a way:

The downward arrows are the leftmost-child links; the sideways arrows are the right-sibling links

Recursions on Trees

Traversal

Traversal is a process of visiting each node in a tree data structure, exactly once, in a systematic way

Pre order is a form of tree traversal, where the action is called firstly on the current node, and then the pre order function is called again recursively on each subtree from left to right
Pre order listing on the figure is: + a − b c d

Post order is a form of tree traversal, where the post order function is called recursively on each subtree from left to right and then the action is called
Post order listing on the figure is: a b c − d * +

Evaluating Expression Tree

There are three types of expressions: infix, prefix and postfix

Infix are expressions in the ordinary notation, where binary operators appear between their operands. For the figure above the infix expression is: a + (b − c) * d. The equivalent prefix and postfix expressions are: + a − b c d for prefix and a b c − d * + for postfix
Having either the prefix or postfix expression the infix expression can be constructed in a simple way. For instance, in order to construct the infix expression from prefix one, firstly the operator that is followed by the required number of operands with no embedded operators must be found
In prefix expression + a * − b c d the − b c is such a string, since the minus sign, like all operators in running example takes two operands. This subexpression is then replaced by a new symbol, say x = − b c. Then, this process of identifying an operator followed by its operands repeated again. Now the expression is: + a * x d. The next subexpression will be y = * x d. This reduces the target expression to + a y, which is lastly converted to just a + y
The remainder of the infix expression a + y then is reconstructed by retracing the steps above. Firstly, the y is substituted to x * d, changing the target expression to: a + (x * d). And lastly the x is replaced by − b c or simply b − c. So the target expression now is: a + ((b − c) * d) or a + (b − c) * d
The conversion from postfix to infix expression uses the same algorithm. The only difference is that firstly the operator, which is preceded by the requisite number of operands must be found in order to decompose the postfix expression

Structural Induction

Structural Induction is a proof method that is used to prove some statement S(T) is true for all trees T

For the basis S(T) must be shown to be true when T consists of a single node. For the induction, T is supposed to be a tree with root r and children c1, c2, …, ck, for some k ≥ 1. If T1, T2, …, Tk are the subtrees of T whose roots are c1, c2, …, ck respectively, then the inductive step is to assume that S(T) is true for all trees T. Structural Induction does not make reference to the exact number of nodes in a tree, except to distinguish the basis (one node) from the inductive step (more than one node)

The general template for Structural Induction has the following outline:

1. Specify the statement S(T) to be proved, where T is a tree
2. Prove the basis, that S(T) is true whenever T is a tree with a single node
3. Set up the inductive step by letting T be a tree with root r and k ≥ 1 subtrees, T1, T2, …, Tk. State that the inductive hypothesis assumed: S(Ti) is true for each of the subtrees Ti, i = 1, 2, …, k
4. Prove that S(T) is true under the assumptions mentioned in step 3

Structural Induction is generally needed to prove the correctness of a recursive program that acts on trees

As an example of using the Structural Induction to prove the correctness of statement the evaluate function (listed above) can be used. This function is applied to a tree T by given a pointer to the root of T as a value of its argument n. It then computes the value of the expression represented by T
The following statement must be proved:
S(T): The value returned by evaluate function when called on the root of T equals the value of the arithmetic expression represented by T
For the basis, T consists of a single node. That is, the argument n is a (pointer to a) leaf. Since the operator field has the value ‘i’ when the node represents and operand, the function succeeds

If the node n is not a (pointer to a) leaf, the inductive hypothesis is that S(T’) is true for each tree T’ rooted at one of the children of n. S(T) then must be proved for the tree T rooted at n
Since operators are assumed to be binary, n has two subtrees. By the inductive hypothesis, the values of value_1 and value_2 are the values of the left and right subtrees. In the figure, value_1 holds the value of T1 and value_2 holds the value of T2
Whatever operator appears at the root, n is applied to the two values: value_1 and value_2. For example, if the root holds, +, then the value returned is value_1 + value_2, as it should be for an expression that is the sum of the expressions of trees T1 and T2
Now, S(T) holds for all expression trees T, and, therefore, the function evaluate correctly evaluates trees that represent expressions

Usage

TreeNode - is the top interface, which represents the basic tree data structures. It describes the basic methods, which are implemented by all the trees

Creation

Suppose you need to create the following tree with String data using array-of-pointer representation:

The following code snippet shows how to build such tree:

// Creating the tree nodes
TreeNode<String> n1 = new ArrayMultiTreeNode<>("n1");
TreeNode<String> n2 = new ArrayMultiTreeNode<>("n2");
TreeNode<String> n3 = new ArrayMultiTreeNode<>("n3");
TreeNode<String> n4 = new ArrayMultiTreeNode<>("n4");
TreeNode<String> n5 = new ArrayMultiTreeNode<>("n5");
TreeNode<String> n6 = new ArrayMultiTreeNode<>("n6");
TreeNode<String> n7 = new ArrayMultiTreeNode<>("n7");

// Assigning tree nodes
n1.add(n2);
n1.add(n3);
n1.add(n4);
n2.add(n5);
n2.add(n6);
n4.add(n7);

Storing And Retrieving Data

Data is stored in a tree node during creation:

// Storing string data during creation
TreeNode<String> node = new ArrayMultiTreeNode<>("data");

// Null values can also be stored:
TreeNode<String> nullNode = new ArrayMultiTreeNode<>(null);

Data can be changed after node creation:

// Setting the data
node.setData("data");

// Getting the data
String data = node.data();

Identifying the Root

// Checking whether the current node is root
boolean result = node.isRoot();

// Getting the reference to the root node
TreeNode<String> root = node.root();

NOTE: Calling root() on the root node returns this root node itself

Identifying the Parent

// Getting the reference to the parent node
TreeNode<String> parent = node.parent();

NOTE: If the current node is root, calling node.parent() returns null

Subtrees

// Retrieving subtrees as a new collection
Collection<? extends TreeNode<String>> subtrees = node.subtrees();

// Checking whether the specified subtree is present
TreeNode<String> subtreeToCheck = new ArrayMultiTreeNode<>("subtreeToCheck");
boolean resultHasSubtree = node.hasSubtree(subtreeToCheck);

// Removing the entire subtree with all of its descendants from node
TreeNode<String> subtreeToDrop = new ArrayMultiTreeNode<>("subtreeToDrop");
boolean resultDropSubtree = node.dropSubtree(subtreeToDrop);

Checking For Presence

// Checking whether the tree node contains the specified node
TreeNode<String> nodeToCheck = new ArrayMultiTreeNode<>("nodeToCheck");
boolean resultContains = node.contains(nodeToCheck);

// Checking whether the tree node contains all of the collection's nodes
Collection<TreeNode<String>> collectionToCheck;
// ... collection initialization skipped
boolean resultContainsAll = node.containsAll(collectionToCheck);

Finding nodes

// Finding the first occurence of the tree node within the tree with the specified data
TreeNode<String> nodeToFind = node.find("dataToFind");

// Finding the collection of all tree nodes within the tree with the specified data
Collection<TreeNode<String>> nodesToFind = node.findAll("dataToFind");

Modification

// Adding new subtree / tree node
TreeNode<String> nodeToAdd = new ArrayMultiTreeNode<>("nodeToAdd");
node.add(nodeToAdd);

// Removing the tree node from entire tree
TreeNode<String> nodeToRemove = new ArrayMultiTreeNode<>("nodeToRemove");
boolean resultRemove = node.remove(nodeToRemove);

// Removing all of the tree nodes specified in the collection from the entire tree
Collection<TreeNode<String>> collectionToRemove;
// ... collection initialization skipped
boolean resultRemoveAll = node.removeAll(collectionToRemove);

// Removing all of the subtrees with all of theirs descendants from the entire tree
node.clear();

Traversal

TraversalAction allows to define an action, which has a single method perform(TreeNode<T>). This method is called during traversal on each node visited

// Creating traversal action
TraversalAction<TreeNode<String>> action = new TraversalAction<TreeNode<String>>() {
	@Override
	public void perform(TreeNode<String> node) {
		System.out.println(node.data()); // any other action goes here
	}
	
	@Override
	public boolean isCompleted() {
	    return false; // return true in order to stop traversing
	}
};

// Traversing pre order
node.traversePreOrder(action);

// Traversing post order
node.traversePostOrder(action);

Iteration

// Iterating over the tree elements using foreach
for (TreeNode<String> node : root) {
	System.out.println(node.data()); // any other action goes here
}

// Iterating over the tree elements using Iterator
Iterator<TreeNode<String>> iterator = root.iterator();
while (iterator.hasNext()) {
	TreeNode<String> node = iterator.next();
}

NOTE: Iteration starts from the root node and goes in a pre ordered manner
Iterator support removing by calling remove() while iterating
Removing any tree node while iterating using foreach throws ConcurrentModificationException

Converting To Collection

// Retrieving all the tree nodes as a pre ordered collection of elements
Collection<? extends TreeNode<String>> preOrderedCollection = node.preOrdered();

// Retrieving all the tree nodes as a post ordered collection of elements
Collection<? extends TreeNode<String>> postOrderedCollection = node.postOrdered();

NOTE: Any modification of the retrieved collection won't cause the correspondent modification in the tree

Additional Useful Methods

// Checking whether the current node is leaf
boolean nodeIsLeaf = node.isLeaf();

// Retrieving the collection of nodes, which connect the current node with its descendants
TreeNode<String> descendant = new ArrayMultiTreeNode<>("descendant");
Collection<? extends TreeNode<String>> path = node.path(descendant);

// Getting the common ancestor
TreeNode<String> anotherNode = new ArrayMultiTreeNode<>("anotherNode");
TreeNode<String> commonAncestor = node.commonAncestor(anotherNode);

// Checking whether the node is the sibling of the current node
TreeNode<String> sibling = new ArrayMultiTreeNode<>("sibling");
boolean nodeIsSibling = node.isSiblingOf(sibling);

// Checking whether the current node is ancestor of another node
TreeNode<String> anotherDescendantNode = new ArrayMultiTreeNode<>("anotherDescendantNode");
boolean nodeIsAncestor = node.isAncestorOf(anotherDescendantNode);

// Checking whether the current node is descendant of another node
TreeNode<String anotherAncestorNode = new ArrayMultiTreeNode<>("anotherAncestorNode");
boolean nodeIsDescendant = node.isDescendantOf(anotherAncestorNode);

// Getting the size of entire tree including the current tree node
int size = node.size();

// Getting the level of the current tree node within the whole tree
int level = node.level();

// Getting the height of the current tree node
int height = node.height();

K-ary (multinode) Trees

MultiTreeNode - interface, which adds additional methods for multi tree node (K-ary) tree data structures:

// Getting the siblings of the current tree node
Collection<? extends MultiTreeNode<String>> siblings = node.siblings();

// Checking whether the current tree node has all of the subtrees specified in the collection
Collection<? extends MultiTreeNode<String>> collectionToCheck;
// ... collection initialization skipped
boolean resultHasSubtrees = node.hasSubtrees(collectionToCheck);

// Adding all of the subtrees from the collection to the current tree node
Collection<? extends MultiTreeNode<String>> collectionToAdd;
// ... collection initialization skipped
boolean resultAddSubtrees = node.addSubtrees(collectionToAdd);

// Removing all of the subtrees specified in the collection from the current tree node
Collection<? extends MultiTreeNode<String>> collectionToRemove;
// ... collection initialization skipped
boolean resultRemoveSubtrees = node.removeSubtrees(collectionToRemove);

K-ary (multi node) trees are represented by MultiTreeNode interface and have 2 implementations:

• ArrayMultiTreeNode - implementation based on the array-of-pointers representation
• LinkedMultiTreeNode - implementation based on the leftmost-child-right-sibling representation

LinkedMultiTreeNode is better optimized for trees with many nodes, whereas ArrayMultiTreeNode is better for large node trees

License

  Copyright 2016 Scalified <http://www.scalified.com>
 
  Licensed under the Apache License, Version 2.0 (the "License");
  you may not use this file except in compliance with the License.
  You may obtain a copy of the License at

      http://www.apache.org/licenses/LICENSE-2.0

  Unless required by applicable law or agreed to in writing, software
  distributed under the License is distributed on an "AS IS" BASIS,
  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  See the License for the specific language governing permissions and
  limitations under the License.

Scalified Links

• Scalified
• Scalified Official Facebook Page
• Scalified Support