Tree - Data Structure

So far we discussed Linear data structures like
stack
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Introduction to trees
• So far we have discussed mainly linear data structures – strings, arrays,
lists, stacks and queues
• Now we will discuss a non-linear data structure called tree.
• Trees are mainly used to represent data containing a hierarchical
relationship between elements, for example, records, family trees and
table of contents.
• Consider a parent-child relationship
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Tree
• A tree is an abstract model of a hierarchical structure that consists of
nodes with a parent-child relationship.
• Tree is a sequence of nodes
• There is a starting node known as a root node
• Every node other than the root has a parent node.
• Nodes may have any number of children
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Some Key Terms:
• Root − Node at the top of the tree is called root.
• Parent − Any node except root node has one edge upward to a node called parent.
• Child − Node below a given node connected by its edge downward is called its child node.
• Sibling – Child of same node are called siblings
• Leaf − Node which does not have any child node is called leaf node.
• Sub tree − Sub tree represents descendants of a node.
• Levels − Level of a node represents the generation of a node. If root node is at level 0, then its next child node
is at level 1, its grandchild is at level 2 and so on.
• keys − Key represents a value of a node based on which a search operation is to be carried out for a node.
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Some Key Terms:
• Degree of a node:
• The degree of a node is the number of children of that node
• Degree of a Tree:
• The degree of a tree is the maximum degree of nodes in a given tree
• Path:
• It is the sequence of consecutive edges from source node to destination node.
• Height of a node:
• The height of a node is the max path length form that node to a leaf node.
• Height of a tree:
• The height of a tree is the height of the root
• Depth of a tree:
• Depth of a tree is the max level of any leaf in the tree
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Characteristics of trees
• Non-linear data structure
• Combines advantages of an ordered array
• Searching as fast as in ordered array
• Insertion and deletion as fast as in linked list
• Simple and fast
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Application
• Directory structure of a file store
• Structure of an arithmetic expressions
• Used in almost every 3D video game to determine what objects need to be
rendered.
• Used in almost every high-bandwidth router for storing router-tables.
• used in compression algorithms, such as those used by the .jpeg and .mp3 file-
formats.
FOR Further detail Click Here
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Introduction To Binary Trees
• A binary tree, is a tree in which no node can have more than two
children.
• Consider a binary tree T, here ‘A’ is the root node of the binary tree T.
• ‘B’ is the left child of ‘A’ and ‘C’ is the right
child of ‘A’
• i.e A is a father of B and C.
• The node B and C are called siblings.
• Nodes D,H,I,F,J are leaf node
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Binary Trees
• A binary tree, T, is either empty or such that
I. T has a special node called the root node
II. T has two sets of nodes LT and RT, called the left subtree and right subtree of
T, respectively.
III. LT and RT are binary trees.
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Binary Tree
• A binary tree is a finite set of elements that are either empty or is
partitioned into three disjoint subsets.
• The first subset contains a single element called the root of the tree.
• The other two subsets are themselves binary trees called the left and right
sub-trees of the original tree.
• A left or right sub-tree can be empty.
• Each element of a binary tree is called a node of the tree.
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The following figure shows a binary tree with 9 nodes where A is the root
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Binary Tree
• The root node of this binary tree is A.
• The left sub tree of the root node, which we denoted by LA, is the set
LA = {B,D,E,G} and the right sub tree of the root node, RA is the set
RA={C,F,H}
• The root node of LA is node B, the root node of RA is C and so on
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Binary Tree Properties
• If a binary tree contains m nodes at level L, it contains at most 2m
nodes at level L+1
• Since a binary tree can contain at most 1 node at level 0 (the root), it
contains at most 2L nodes at level L.
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Types of Binary Tree
• Complete binary tree
• Strictly binary tree
• Almost complete binary tree
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Strictly binary tree
• If every non-leaf node in a binary tree has nonempty left and right sub-trees, then
such a tree is called a strictly binary tree.
• Or, to put it another way, all of the nodes in a strictly binary tree are of degree zero
or two, never degree one.
• A strictly binary tree with
N leaves always contains 2N – 1 nodes.
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Complete binary tree
• A complete binary tree is a binary tree in which every level, except possibly the last, is completely
filled, and all nodes are as far left as possible.
• A complete binary tree of depth d is called strictly binary tree if all of whose leaves are at level d.
• A complete binary tree has 2d nodes at every depth d and 2d -1 non leaf nodes
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Almost complete binary tree
• An almost complete binary tree is a tree where for a right child, there is always a
left child, but for a left child there may not be a right child.
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Tree traversal
• Traversal is a process to visit all the nodes of a tree and may print their
values too.
• All nodes are connected via edges (links) we always start from the root
(head) node.
• There are three ways which we use to traverse a tree
• In-order Traversal
• Pre-order Traversal
• Post-order Traversal
• Generally we traverse a tree to search or locate given item or key in the tree
or to print all the values it contains.
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Pre-order, In-order, Post-order
• Pre-order
<root><left><right>
• In-order
<left><root><right>
• Post-order
<left><right><root>
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Pre-order Traversal
• The preorder traversal of a nonempty binary tree is defined as follows:
• Visit the root node
• Traverse the left sub-tree in preorder
• Traverse the right sub-tree in preorder
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Pre-order Pseudocode
struct Node{
char data;
Node *left;
Node *right;
}
void Preorder(Node *root)
{
if (root==NULL) return;
printf (“%c”, root->data);
Preorder(root->left);
Preorder(root->right);
}
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In-order traversal
• The in-order traversal of a nonempty binary tree is defined as follows:
• Traverse the left sub-tree in in-order
• Traverse the right sub-tree in inorder
• The in-order traversal output
of the given tree is
H D I B E A F C G
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In-order Pseudocode
struct Node{
char data;
Node *left;
Node *right;
}
void Inorder(Node *root)
{
Inorder(root->left);
Inorder(root->right);
}
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Post-order traversal
• The in-order traversal of a nonempty binary tree is defined as follows:
• Traverse the left sub-tree in post-order
• Traverse the right sub-tree in post-order
• The in-order traversal output
of the given tree is
H I D E B F G C A
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Post-order Pseudocode
struct Node{
char data;
Node *left;
Node *right;
}
void Postorder(Node *root)
{
Postorder(root->left);
Postorder(root->right);
}
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Binary Search Tree(BST)
• A binary search tree (BST) is a binary tree that is either empty or in
which every node contains a key (value) and satisfies the following
conditions:
• All keys in the left sub-tree of the root are smaller than the key in the root
node
• All keys in the right sub-tree of the root are greater than the key in the root
node
• The left and right sub-trees of the root are again binary search trees
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• A binary search tree is basically a binary tree, and therefore it can be
traversed in inorder, preorder and postorder.
• If we traverse a binary search tree in inorder and print the identifiers
contained in the nodes of the tree, we get a sorted list of identifiers in
ascending order.
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Why Binary Search Tree?
• Let us consider a problem of searching a list.
• If a list is ordered searching becomes faster if we use contiguous
list(array).
• But if we need to make changes in the list, such as inserting new
entries or deleting old entries, (SLOWER!!!!) because insertion and
deletion in a contiguous list requires moving many of the entries every
time.
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Why Binary Search Tree?
• So we may think of using a linked list because it permits insertion and
deletion to be carried out by adjusting only few pointers.
• But in an n-linked list, there is no way to move through the list other
than one node at a time, permitting only sequential access.
• Binary trees provide an excellent solution to this problem. By making
the entries of an ordered list into the nodes of a binary search tree, we
find that we can search for a key in O(logn)
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Time Complexity
Array Linked List BST
Search O(n) O(n) O(logn)
Insert O(1) O(1) O(logn)
Remove O(n) O(n) O(logn)
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Operations on Binary Search Tree (BST)
• Following operations can be done in BST:
• Search(k, T): Search for key k in the tree T. If k is found in some node of tree
then return true otherwise return false.
• Insert(k, T): Insert a new node with value k in the info field in the tree T such
that the property of BST is maintained.
• Delete(k, T):Delete a node with value k in the info field from the tree T such
that the property of BST is maintained.
• FindMin(T), FindMax(T): Find minimum and maximum element from the
given nonempty BST.
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Searching Through The BST
• Compare the target value with the element in the root node
 If the target value is equal, the search is successful.
If target value is less, search the left subtree.
If target value is greater, search the right subtree.
If the subtree is empty, the search is unsuccessful.
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Insertion of a node in BST
• To insert a new item in a tree, we must first verify that its key is different
from those of existing elements.
• If a new value is less, than the current node's value, go to the left subtree,
else go to the right subtree.
• Following this simple rule, the algorithm reaches a node, which has no left
or right subtree.
• By the moment a place for insertion is found, we can say for sure, that a
new value has no duplicate in the tree.
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Algorithm for insertion in BST
• Check, whether value in current node and a new value are equal. If so,
duplicate is found. Otherwise,
• if a new value is less, than the node's value:
• if a current node has no left child, place for insertion has been found;
• otherwise, handle the left child with the same algorithm.
• if a new value is greater, than the node's value:
• if a current node has no right child, place for insertion has been found;
• otherwise, handle the right child with the same algorithm.
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Deleting a node from the BST
• While deleting a node from BST, there may be three cases:
1. The node to be deleted may be a leaf node:
• In this case simply delete a node and set null pointer to its parents those side
at which this deleted node exist.
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2. The node to be deleted has one child
• In this case the child of the node to be deleted is appended to its parent node.
Suppose node to be deleted is 18
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Huffman Algorithm
• Huffman algorithm is a method for building an extended binary tree
with a minimum weighted path length from a set of given weights.
• This is a method for the construction of minimum redundancy codes.
• Applicable to many forms of data transmission
• multimedia codecs such as JPEG and MP3
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Huffman Algorithm
• 1951, David Huffman found the “most efficient method of representing
numbers, letters, and other symbols using binary code”. Now standard
method used for data compression.
• In Huffman Algorithm, a set of nodes assigned with values if fed to the
algorithm. Initially 2 nodes are considered and their sum forms their parent
node.
• When a new element is considered, it can be added to the tree.
• Its value and the previously calculated sum of the tree are used to form the
new node which in turn becomes their parent.
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Huffman Algorithm
• Let us take any four characters and their frequencies, and sort this list by
increasing frequency.
• Since to represent 4 characters the 2 bit is sufficient thus take initially two
bits for each character this is called fixed length character.
• Here before using Huffman algorithm the total number of bits required is:
nb=3*2+5*2+7*2+10*2 =06+10+14+20 =50bits
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character frequencies
E 10
T 7
O 5
A 3
Character frequencies code
A 3 00
O 5 01
T 7 10
E 10 11
sort

• Thus after using Huffman algorithm the total number of bits required is
nb=3*3+5*3+7*2+10*1 =09+15+14+10 =48bits
i.e
(50-48)/50*100%=4%
Since in this small example we save about 4% space by using Huffman algorithm. If we take large
example with a lot of characters and their frequencies we can save a lot of space
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Character frequencies code
A 3 110
O 5 111
T 7 10
E 10 0

• Lets say you have a set of numbers and their frequency of use and
want to create a huffman encoding for them
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Value Frequencies
1 5
2 7
3 10
4 15
5 20
6 45
Huffman Algorithm

Huffman Algorithm
• Creating a Huffman tree is simple. Sort this list by frequency and make the two-lowest elements
into leaves, creating a parent node with a frequency that is the sum of the two lower element's
frequencies:
12:*
/
5:1 7:2
• The two elements are removed from the list and the new parent node, with frequency 12, is
inserted into the list by frequency. So now the list, sorted by frequency, is:
10:3
12:*
15:4
20:5
45:6
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Huffman Algorithm
• You then repeat the loop, combining the two lowest elements. This results in:
22:*
/
10:3 12:*
/
5:1 7:2
• The two elements are removed from the list and the new parent node, with frequency 12, is
inserted into the list by frequency. So now the list, sorted by frequency, is:
15:4
20:5
22: *
45:6
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Huffman Algorithm
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After sorted
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Value Z K M C L D U E
Frequency 2 7 24 32 42 42 37 120
Value C D E K L M U Z
Frequency 32 42 120 7 42 24 37 2
Huffman Algorithm
We can represent it using 3 bit

Huffman Algorithm
• Find code for
• DEED
• MUCK
• Try to decode the bit string
1011001110111101
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Assignments
• Slides at myblog
http://www.ashimlamichhane.com.np/2016/07/tree-slide-for-data-
structure-and-algorithm/
• Assignments at github
https://github.com/ashim888/dataStructureAndAlgorithm/tree/dev/As
signments/assignment_7
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Reference
• https://www.siggraph.org/education/materials/HyperGraph/video/mp
eg/mpegfaq/huffman_tutorial.html
• https://en.wikipedia.org/wiki/Binary_search_tree
• https://www.cs.swarthmore.edu/~newhall/unixhelp/Java_bst.pdf
• https://www.cs.usfca.edu/~galles/visualization/BST.html
• https://www.cs.rochester.edu/~gildea/csc282/slides/C12-bst.pdf
• http://www.tutorialspoint.com/data_structures_algorithms/tree_data
_structure.htm
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Tree - Data Structure

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