MODULE 4
TREES

CONTENTS
 Terminology
 Binary Trees
 Properties of Binary trees
 Array and linked representation of Binary Trees
 Binary Tree Traversals - Inorder, postorder, preorder
 Additional Binary tree operations
 Threaded binary trees
 Binary Search Trees
 Definition
 Insertion
 Deletion
 Traversal
 Searching
 Application of Trees-Evaluation of Expression
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 2

TERMINOLOGY
TREE
“A Tree is a finite set of one or more nodes such that
(i) There is a specially designated node called the ROOT.
(ii) The remaining nodes are partitioned into n ≥ 0 disjoint sets
𝑇1,…., 𝑇𝑛, where each of these sets is a tree. 𝑇1,…., 𝑇𝑛 are
called Subtrees of the root.”
A
B
C D
E
F
G H
TRE
ROO
T
𝑻𝟏
𝑻𝟐
𝑻𝟑
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 3

TERMINOLOGY
NODE
“A Node is an item of information.”
 A node is a component of a tree that contains data.
 Various nodes of a tree are connected to one another by
Edges of the tree.
DEGREE
“The Degree of a node is the number of subtrees of that
node.”
LEAF or TERMINAL NODE
“A Leaf or Terminal node is a node whose degree is zero.”
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 4

TERMINOLOGY
CHILDREN
“ The roots of the subtrees of a node X are called Children of
X.”
PARENT
“A node having a left and/or right subtree is said to be the
Parent of the left and/or right subtree.”
SIBLINGS
“Children nodes of the same parent are called Siblings.”
DEGREE OF A TREE
“The Degree of a Tree is the maximum of the degree of the
nodes in the tree.”
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 5

TERMINOLOGY
ANCESTOR
“The Ancestor of a node are all the nodes along the path
from the root to that node.”
DESCENDANT
“All the nodes that are reachable from a particular node while
moving downwards are called Descendants of that node.”
 All the nodes that are reachable from the left side of a node are
called its Left Descendants.
 All the nodes that are reachable from the right side of a node
are called its Right Descendants.
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 6

TERMINOLOGY
LEVEL
“The Level of a particular node in a tree is the distance
between the root node and that node.”
 The root node is always at level 0. However, some authors
consider the level of root node to be 1.
HEIGHT or DEPTH
“The Height or Depth of a tree is the maximum level of any
node in the tree.”
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 7

TERMINOLOGY
Representation of trees
A tree can be represented in 3 different ways. They are:
 List representation
 Left – Child, Right – Sibling representation
 Representation as a degree two tree
List representation
 In this approach, every tree node is represented as a linked list
node.
 For a given node, all its children nodes will appear to its right
side.
 If the child node has its own children, then this will be
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 8

TERMINOLOGY
 The above tree is first written as a list of nodes as follows,
( A ( B( E(K, L), F), C(G), D( H(M), I, J)))
 We now represent the above tree as a linked list. We start with
node A.
A
B
E F
C D
H J
G I
K L M
A
ROO
T
B
1st Child
C
2nd Child
D
3rd Child
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 9

TERMINOLOGY
 Since nodes B, C and D have their own children, they must be
represented as separate lists. The resulting list representation
of the above tree is,
A
B
E F
C D
H J
G I
K L M
A
ROO
T
1st
2nd
0
3rd
B F 0
E K L 0
1st 2nd
1st 2nd
C G 0
1st
D I
1st 2nd
H M 0
1st
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 10

TERMINOLOGY
Left – Child, Right – Sibling representation
 For this representation, we scan every node of a given tree and
check if that node has either a left child or right sibling or both.
 If a node has either a left child or right sibling or both, we
establish the relationship in the representation and move on to
the next node.
 Consider the following tree,
A
B
E F
C D
H J
G I
K L M
Left – Child, Right –
Sibling
A
B
E F
C D
H J
G I
K L M
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 11

TERMINOLOGY
Degree two tree Representation(Binary Tree)
 This representation is also known as a Left Child – Right
Child representation.
 This representation for a tree is obtained in two steps.
STEP 1 – For a given tree, get its Left Child – Right Sibling
representation.
STEP 2 – Rotate the horizontal edges in the clockwise
direction by 45 degrees.
 The resulting tree is a Degree two representation for the given
tree.
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 12

TERMINOLOGY
A
B
E F
C D
H J
G I
K L M
Left – Child, Right –
Sibling
A
B
E F
C D
H J
G I
K L M
Degree Two
A
B
E F C D
H J
G I
K L
M
A
B
E F C
D
H J
G
I
K L
M
A
B
E
F
C
D
H J
G
I
K L
M
A
B
E
F
C
D
H
J
G
I
K L
M
A
B
E
F
C
D
H
J
G
I
K L
M
A
B
E
F
C
D
H
J
G
I
K
L
M
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 13

BINARY TREES
“A Binary Tree is a finite set of nodes that is either empty
or consists of a root and two disjoint binary trees called the left
subtree and the right subtree.”
 Every node in a binary tree can have either no child or a max
of two children(left and right child).
 A typical node of a binary tree is represented as follows,
1000
llink data rlink
Address of
the left
subtree
Address of
the right
subtree
Binary Tree Node
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 14

BINARY TREES
The following example trees present some important properties of
Binary trees.
NULL
root
A Binary tree
can be empty
10
root
A Binary tree
node can have
no child
10
root
20
A Binary tree
node can have
one child
A Binary tree node
can have two
children
10
root
20 30
10
root
20 30
40
A Binary tree node
cannot have more than
two children
10
root
20 30
A Binary tree
cannot have a
cycle
10
root
20 30
A Binary tree cannot
have bidirectional
edges
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 15

BINARY TREES
Following are some points of difference between a tree and a
binary tree.
Tree Binary Tree
A Tree cannot be empty. A Binary tree can be empty
Each node can have many children
or nodes.
Each node can have at most two
children.
There is no distinction between the
order of the children nodes in a tree.
Both the trees shown below are the
same.
In a binary tree, we distinguish
between the order of the children.
The trees shown below are different.
10
root
20
10
root
20
10
root
20
10
root
20
Left
child
Right child
chil
d
chil
d
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 16

BINARY TREES
ADT Binary_Tree(BinTree) is
objects: a finite set of nodes either empty or consisting of a root node,
left BinTree and right BinTree.
functions: for all bt, bt1, bt2 Є BinTree, item Є element
BinTree Create() ::= creates an empty binary tree
Boolean IsEmpty(bt) ::= if(bt == Create()) return TRUE, else return
FALSE
BinTree MakeBT(bt1, item, bt2) ::= returns a binary tree where bt1
and bt2 are the left and right subtrees
and item is the data in the root node.
BinTree Lchild(bt) ::= if ‘bt’ is empty, return ERROR, else returns left
subtree of ‘bt’.
BinTree Data(bt) ::= if ‘bt’ is empty, return ERROR, else returns root
node of ‘bt’.
BinTree Rchild(bt) ::= if ‘bt’ is empty, return ERROR, else returns right
subtree of ‘bt’.
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 17

PROPERTIES OF BINARY TREE
Maximum number of nodes
a) The maximum number of nodes on level i of a binary tree is 𝟐𝒊−𝟏
,
for i ≥ 1.
Proof
 We prove the above property by induction on i.
Induction Base
 Let i = 1. This corresponds to the first level.
 Substituting the value of i=1 in the given property we get,
no. of nodes at level i = 𝟐𝒊−𝟏 = 𝟐𝟏−𝟏 = 𝟐𝟎 = 1
 This means at the first level, there is only one node.
 This is true as the first level contains only one node, which is the root.
Hence the induction base is proved.
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 18

PROPERTIES OF BINARY TREE
Induction Hypothesis
 Let us consider i to be a positive integer greater than 1.
 This means, i now represents a level other than the first level.
 From the given expression, we can say that the maximum number of
nodes at level i-1 is,
no. of nodes at level (i-1) = 𝟐 𝒊−𝟏 −𝟏
= 𝟐𝒊−𝟏−𝟏
= 𝟐𝒊−𝟐
 With the help of this hypothesis, we now prove the property.
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 19

PROPERTIES OF BINARY TREE
Induction Step
 From the induction hypothesis we have,
no. of nodes at level (i-1) = 𝟐𝒊−𝟐
 Now consider the following binary tree.
 The maximum number of nodes at level 1 = 1
 The maximum number of nodes at level 2 = 2
 The maximum number of nodes at level 3 = 4
 The maximum number of nodes at level 4 = 8
a
root
b c
d e f g
h i j k l m n o
Level
1
Level
2
Level
3
Level
4
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 20

PROPERTIES OF BINARY TREE
 From the previous set of statements, we can now generalize
that, the maximum number of nodes at level i in a binary
tree is twice the maximum number of nodes at level i-1.
 This can be written as,
max. no. of nodes at level i = 2 X max. no. of nodes at level
(i-1)
= 2 X 2i−2
= 2i−2 + 1
max. no. of nodes at level i = 𝟐𝒊−𝟏
Hence proof of the property.
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 21

PROPERTIES OF BINARY TREE
b) The maximum number of nodes in a binary tree of depth K
is 𝟐𝑲 − 𝟏, for K ≥ 1.
a
root
b c
d e f g
h i j k l m n o
Level
1
Level
2
Level
3
Level
4
 In the above tree,
max. number of nodes at level 1 = 20
max. number of nodes at level 2 = 21
max. number of nodes at level 3 = 22
…..
max. number of nodes at level i = 2𝑖−1
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 22

PROPERTIES OF BINARY TREE
 Hence, we can say that,
max. no. of nodes in the binary tree = 𝟐𝟎 + 𝟐𝟏 + 𝟐𝟐 + …… +
𝟐𝒊−𝟏
 The above sum is a geometric progression and can be solved
as,
S = a (𝒓𝒏
- 1) / (r - 1) (1)
Where,
a – initial term
r – common ratio
n – total number of terms
For our problem,
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 23

PROPERTIES OF BINARY TREE
 Substituting the values in (1) , we get,
S = 𝟐𝟎
(𝟐𝒊
- 1) / (2 - 1) = 𝟐𝒊
- 1
 Since S is the sum that corresponds to the maximum number
of nodes in the binary tree, we can write the above expression
as,
max. no. of nodes in the binary tree = 𝟐𝒊 - 1 (2)
 The depth of a tree is denoted by K. Depth of a tree is the
maximum level of the tree.
 In our example, the maximum level is denoted by i. Hence,
K = i
 Making this substitution in (2) we have,
max. no. of nodes in the binary tree of depth K = 𝟐𝑲 - 1
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 24

PROPERTIES OF BINARY TREE
Relation between number of leaf nodes and
degree-2 nodes
For any non-empty binary tree, T, if 𝒏𝟎 is the number of leaf
nodes and 𝒏𝟐 is the number of degree-2 nodes, then,
𝒏𝟎 = 𝒏𝟐 + 1
Proof
 Let us consider the number of nodes with degree 1 to be 𝒏𝟏.
 The total number of nodes in the tree is given by,
n = 𝒏𝟎 + 𝒏𝟏 + 𝒏𝟐 (1)
 We next establish the relationship between the number of
nodes and branches in a given binary tree.
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 25

PROPERTIES OF BINARY TREE
 In the above tree, there are 12 branches and 13 nodes.
 Hence we can define the following relationship between the
number of nodes(n) and number of branches(B) for a given
binary tree.
n = B + 1 (2)
 Next we derive an expression for the number of branches B.
a
root
b c
d e f g
i j k l m n
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 26

PROPERTIES OF BINARY TREE
 For a given binary tree,
Total number of branches of degree 1 nodes = 𝒏𝟏 (3)
Total number of branches of degree 2 nodes = 𝟐𝒏𝟐 (4)
 Hence, the total number of branches , B, in a given binary tree
is,
B = 𝒏𝟏 + 𝟐𝒏𝟐 (5)
a
root
b c
d e f g
i j k l m n
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 27

PROPERTIES OF BINARY TREE
 From equation (2) we have,
n = B + 1
 Substituting (5) in the above equation, we have,
n = 𝒏𝟏 + 𝟐𝒏𝟐 + 1 (6)
 Subtracting (6) from (1), we have,
n – n = 𝒏𝟏 + 𝟐𝒏𝟐 + 1 – ( 𝒏𝟎+ 𝒏𝟏+ 𝒏𝟐)
0 = 𝒏𝟏 + 𝟐𝒏𝟐 + 1 – 𝒏𝟎 - 𝒏𝟏 - 𝒏𝟐
0 = 𝒏𝟐 + 1 – 𝒏𝟎
𝒏𝟎 = 𝒏𝟐 + 1
Hence the proof of the property
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 28

TYPES OF BINARY TREES
Strictly Binary Tree
“A Strictly Binary Tree is a binary tree where every node
except the leaf nodes must have two children.”
 A Strictly Binary Tree is also known as a Full Binary Tree or
Proper Binary Tree.
 Example
a
root
b c
d e f g
h i j k l m n o
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 29

TYPES OF BINARY TREES
Complete Binary Tree
“A Complete Binary Tree is a binary tree wherein every
level except possibly the last level is completely filled.”
 If the last level of the tree is not completely filled, then the
nodes in that level must be filled from left to right.
 Examples
a
roo
t
b c
a
roo
t
b c
d
a
roo
t
b c
d e f
a
roo
t
b c
e f
a
roo
t
b c
d f
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 30

TYPES OF BINARY TREES
Skewed Tree
“A Skewed Tree is a binary tree that consists of either only
left subtrees or right subtrees.”
 This means, either all nodes of the tree will only have left child
or only right child.
 Example
a
roo
t
b
d
a
roo
t
b
d
Left Skewed Tree Right Skewed
Tree
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 31

TYPES OF BINARY TREES
Almost Complete Binary Tree
“An Almost Complete Binary Tree is a special kind of
binary tree where insertion takes place level by level and from left
to right order at each level and the last level is not filled fully
always.”
a
roo
t
b c
d e f g
a
roo
t
b c
d e f
Complete
Binary Tree
Almost
Complete
Binary Tree
 In an Almost Complete binary tree, the last level is never full.
 An Almost Complete binary tree is always a complete binary tree.
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 32

BINARY TREE REPRESENTATION
There are two ways by which a binary tree can be represented.
(i) Array Representation
(ii) Linked Representation
Array Representation
 In this approach, every node of the binary tree is represented
as an array element.
 A one dimensional array is used for this purpose.
 To create this representation, we first number the nodes of the
given binary tree in a sequence.
 These numbers will then become the array indices for the
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 33

BINARY TREE REPRESENTATION
 Consider the following binary tree.
a
1
b c
d e f g
h i j k l m n o
 We start by numbering the nodes of the
above tree sequentially.
 Next, we create an array and place the
nodes in the array at their respective
index positions.
2 3
4 5 6 7
15
14
13
12
11
10
9
8
--
a
b
c
d
e
f
g
h
i
j
k
l
m
n
o
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 34

BINARY TREE REPRESENTATION
 Another example
a
1
b c
d f g
h l o
2 3
4 6 7
15
12
8
STEP 1 – Number the nodes sequentially.
STEP 2 – Create an array and place the
nodes at the respective index
positions.
--
a
b
c
d
--
f
g
h
--
--
--
l
--
--
o
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 35

BINARY TREE REPRESENTATION
Array Representation Property
“ If a complete binary tree with n nodes is represented
sequentially, then for any node with the index i, 1 ≤ i ≤ n, we
have,
(1) parent(i) is at index 𝒊/𝟐 if i ≠ 1.
(2) leftchild(i) is at index 2i if 2i < n. If 2i > n, then i has no left
child.
(3) rightchild(i) is at index 2i+1 if 2i+1 < n. If 2i+1 > n, then i
has no right child.”
a
1
b c
d e f g
2 3
4 5 6 7
--
a
b
c
d
e
f
g
0
1
2
3
4
5
6
7
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 36

BINARY TREE REPRESENTATION
Disadvantage of Array Representation
The biggest disadvantage of array representation is that, if the
given binary tree is not full or complete, there is a lot of wastage
of space.
a
1
b c
d e f g
2 3
4 5 6 7
a
1
c
g
3
7
-- a b c d e f g
0 1 2 3 4 5 6 7
-- a -- c -- -- -- g
0 1 2 3 4 5 6 7
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 37

BINARY TREE REPRESENTATION
Linked Representation
 In this representation, every tree node is represented as a
doubly linked list node.
 Every node has 3 fields namely data, leftchild, rightchild.
 The C definition of a binary tree node in linked representation
is,
struct node{
int data;
struct node *leftchild;
struct node
*rightchild;
};
LeftChil
d
Data RightChi
ld
treepointe
r
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 38

BINARY TREE REPRESENTATION
 Examples
a
b
c
d e
a
0 c 0
b
0 d 0 0 e 0
root
a
b
c
d
a 0
root
b 0
c 0
0 d 0
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 39

BINARY TREE TRAVERSALS
 Traversal of a tree is the process of visiting each node of a tree
exactly once and then performing some operation on that
node, such as displaying the contents of that node.
 The traversal of a tree results in a linear order of the nodes in
the tree.
 There are 3 important traversal techniques for a binary tree:
 Inorder (LVR)
 Preorder (VLR)
 Postorder (LRV)
 Here L means move left, V means visit the node, R means
move right.
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 40

D
+
E
*
*
C
/
A B
BINARY TREE TRAVERSALS
Inorder Traversal
For every node in the binary tree, we perform the following steps:
STEP 1 – Move to the left child of the node
STEP 2 – Visit the node(Print the node data)
STEP 3 – Move to the right child of the node
L V
R
L V
R L V
R
L V
R L V
R
L V
R L V
R
L V
R
L V R
1 L V
R
2 L V
R
3 L V R
4 L V
R
5 L V
R
5 L V
R
5 L V
R
4 L V
R
4 L V
R
8 L V R
8 L V R
8 L V R
3 L V R
3 L V R
11 L V
R
11 L V
R
11 L V
R
2 L V
R
2 L V
R
14 L V
R
14 L V
R
14 L V
R
1 L V
R
1 L V
R
17 L V
R
17 L V
R
17 L V
R
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 41

BINARY TREE TRAVERSALS
 The C function for inorder traversal is presented below.
void inorder( treepointer *ptr){
if(ptr != NULL){
inorder(ptr ->leftchild);
printf(“%d”, ptr -
>data);
inorder(ptr -
>rightchild);
}
}
+
0 b 0
0 a 0
ptr
1
2 3
inorder(1
)
inorder(2
)
inroder(0)
a
inroder(0)
inorder(3
)
inroder(0)
b
inroder(0)
+
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 42

BINARY TREE TRAVERSALS
Preorder Traversal
For every node in the binary tree, we perform the following steps:
STEP 1 – Visit the node(Print the node data)
STEP 2 – Move to the left child of the node
STEP 3 – Move to the right child of the node
D
+
E
*
*
C
A
V L R
V L R
V L R
V L R
V L R
V L R
V L R
1 V L
R
1 V L
R
2 V L
R
2 V L
R
3 V L
R
3 V L
R
4 V L
R
4 V L
R
4 V L
R
3 V L
R
7 V L
R
7 V L
R
7 V L
R
2 V L
R
10 V L R
10 V L R
10 V L R
1 V L
R
13 V L
R
13 V L
R
13 V L
R
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 43

BINARY TREE TRAVERSALS
 The C function for preorder traversal is presented below.
void preorder( treepointer *ptr){
if(ptr != NULL){
printf(“%d”, ptr -
>data);
preorder(ptr -
>leftchild);
preorder(ptr -
>rightchild);
}
}
+
0 b 0
0 a 0
ptr
1
2 3
preorder(
1)
+
preorder(2
)
a
preroder(0)
preroder(0)
preorder(3
)
b
preroder(0)
preroder(0)
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 44

BINARY TREE TRAVERSALS
Postorder Traversal
For every node in the binary tree, we perform the following steps:
STEP 1 – Move to the left child of the node
STEP 2 – Move to the right child of the node
STEP 3 – Visit the node(Print the node data)
D
+
E
*
*
C
A
L R V
L R V
L R V
L R V L R V
L R V
L R V
1 L R
V
2 L R
V
3 L R
V
4 L R
V
4 L R
V
4 L R
V
3 L R
V
7 L R
V
7 L R
V
7 L R
V
3 L R
V
2 L R
V
10 L R V
10 L R V
10 L R V
2 L R
V
1 L R
V
13 L R
V
13 L R
V
13 L R
V
1 L R
V
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 45

BINARY TREE TRAVERSALS
 The C function for preorder traversal is presented below.
void postorder( treepointer *ptr){
if(ptr != NULL){
postorder(ptr -
>leftchild);
postorder(ptr -
>rightchild);
printf(“%d”, ptr ->data);
}
}
+
0 b 0
0 a 0
ptr
1
2 3
postorder(
postorder(
2)
postorder(
0)
postorder(
0)
a
postorder(
3)
postorder(
0)
postorder(
0)
b
+
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 46

BINARY TREE TRAVERSALS
Iterative Inorder Traversal
 So far, we have seen the different traversals using recursion.
 In this section, we attempt to implement the Inorder traversal
using iteration.
 We start by defining the structure that represents a node of the
tree.
struct node{
int data;
struct node *left, *right;
};
typedef struct node NODE;
 We now implement Iterative Inorder Traversal using Stacks.
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 47

BINARY TREE TRAVERSALS
+
0 b 0
0 a 0
root
100
200 300
+
a b
root
S[20
]
19
…
3
2
1
0
cur =
top = -1
Top
cur = 100
top = 0
Top
100
cur = 200
cur
+
0 b 0
0 a 0
root
100
200 300
cur
top = 1 Top
200
100
cur = NULL
cur
cur = 200
Top
top = 0
100
Output
a
cur = NULL
cur
cur
cur = 100
cur
top = -1
Top
Output
a +
cur = 300
cur
cur
top = 0
Top
300
cur = NULL
cur
cur = 300
cur
top = -1
Top
Output
a + b
cur = NULL
cur
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 48

BINARY TREE TRAVERSALS
Level Order Traversal
 In this technique, the nodes in the tree are traversed level by
level.
 For this kind of traversal, we make use of a Circular Queue.
a
b c
ptr
d e f g
a
c
b
ptr
20 30
d
40
e
50
f
60
g
70
10
0 1 2 3 4 5 6 7 … 19
queue[20
]
f = 0 r = 0 ptr =
Output
a
10
r = 1
f = 1
20
r = 2
20 30
r = 3
f = 2
30 Output
ab
r = 4
30 40
r = 5
30 40 50
ptr =
f = 3
40 50
ptr =
Output
abc
r = 6
40 50 60
r = 7
40 50 60 70
f = 4 ptr =
50 60 70 Output
abcd
r = 8
50 60 70 0
r = 9
50 60 70 0 0
f = 5 ptr =
60 70 0 0 Output
abcde
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 49

BINARY TREE TRAVERSALS
Constructing Binary Trees from Traversal
sequence
 In this section, we see how a binary tree can be constructed for
a given traversal sequence.
 It is possible to construct a binary tree from the traversals,
provided we are given a combination of,
 Inorder and Preorder traversal sequence
 Inorder and Postorder traversal sequence
 Preorder or Postorder traversal sequence is used to
determine the root node from the given sequence of
nodes.
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 50

BINARY TREE TRAVERSALS
Example 1
Inorder : D B E A F C
Preorder : A B D E C F
A
DBE FC
A
FC
B
D E
A
B
D E
C
F
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 51

BINARY TREE TRAVERSALS
Example 2
Inorder : D B E A F C
Postorder : D E B F C A
A
DBE FC
A
DBE C
F
A
B
D E
C
F
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 52

a
temp
10
ADDITIONAL BINARY TREE OPERATIONS
Copying Binary
Trees
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 53
2 a 3
0 c 0
0 b 0
original
1
2 3
copy(1)
copy(1)
templeft = copy(2)
2 a 3
0 c 0
0 b 0
original
1
2 3
b
temp
20
a
10
copy(1)
templeft = copy(2)
templeft =
copy(0)
2 a 3
0 c 0
0 b 0
1
2 3
copy(1)
templeft = copy(2)
templeft = 0
0 b
temp
20
a
10
copy(1)
templeft = copy(2)
templeft = 0
tempright =
copy(0)
original 2 a 3
0 c 0
0 b 0
1
2 3
copy(1)
templeft = copy(2)
templeft = 0
tempright = 0
0 b 0
temp
20
a
10
copy(1)
templeft = 20
0 b 0
temp
20
20
a
10
original
copy(1)
templeft = 20
tempright = copy(3)
2 a 3
0 c 0
0 b 0
1
2 3
original
0 b 0
tem
p
20
20
a
c
30
10
copy(1)
templeft = 20
tempright = copy(3)
templeft =
copy(0)
2 a 3
0 c 0
0 b 0
1
2 3
copy(1)
templeft = 20
tempright = copy(3)
templeft = 0
0 c
copy(1)
templeft = 20
tempright = copy(3)
templeft = 0
tempright=copy(0)
original
2 a 3
0 c 0
0 b 0
1
2 3
copy(1)
templeft = 20
tempright = copy(3)
templeft = 0
tempright= 0
0 c 0
copy(1)
templeft = 20
tempright = 30
0 b 0
tem
p
20
20
a 30
30
10
0 c 0
10

ADDITIONAL BINARY TREE OPERATIONS
Testing
Equality
3/27/2023
Dr. K. Balakrishnan, 18CS32, Dept. of
CSE, SaIT 54
2 a 3
0 c 0
0 b 0
1
2 3
0 b 0
r2
20
20
a 30
30
10
0 c 0
r1
equal(1, 10)
equal(1, 10)
return (
equal(1, 10)
return (
TRUE &&
equal(1, 10)
return (
TRUE &&
equal(2, 20)
2 a 3
0 c 0
0 b 0
1
2 3
0 b 0
r2
20
20
a 30
30
10
0 c 0
r1
equal(1, 10)
return (
TRUE &&
equal(2, 20)
return(
equal(1, 10)
return (
TRUE &&
equal(2, 20)
return(
TRUE &&
equal(1, 10)
return (
TRUE &&
equal(2, 20)
return(
TRUE &&
equal(0, 0)
2 a 3
0 c 0
0 b 0
1
2 3
0 b 0
r2
20
20
a 30
30
10
0 c 0
r1
equal(1, 10)
return (
TRUE &&
equal(2, 20)
return(
TRUE &&
TRUE &&
equal(1, 10)
return (
TRUE &&
equal(2, 20)
return(
TRUE &&
TRUE &&
equal(0, 0);
2 a 3
0 c 0
0 b 0
1
2 3
0 b 0
r2
20
20
a 30
30
10
0 c 0
r1
equal(1, 10)
return (
TRUE &&
equal(2, 20)
return(
TRUE &&
TRUE &&
TRUE );
equal(1, 10)
return (
TRUE &&
TRUE &&
2 a 3
0 c 0
0 b 0
1
2 3
0 b 0
r2
20
20
a 30
30
10
0 c 0
r1
equal(1, 10)
return (
TRUE &&
TRUE &&
equal(3, 30)
2 a 3
0 c 0
0 b 0
1
2 3
0 b 0
r2
20
20
a 30
30
10
0 c 0
r1
equal(1, 10)
return (
TRUE &&
TRUE &&
equal(3, 30)
return (
TRUE &&
equal(1, 10)
return (
TRUE &&
TRUE &&
equal(3, 30)
return (
TRUE &&
equal(0, 0)
2 a 3
0 c 0
0 b 0
1
2 3
0 b 0
r2
20
20
a 30
30
10
0 c 0
r1
equal(1, 10)
return (
TRUE &&
TRUE &&
equal(3, 30)
return (
TRUE &&
TRUE &&
equal(1, 10)
return (
TRUE &&
TRUE &&
equal(3, 30)
return (
TRUE &&
TRUE &&
equal(0, 0)
2 a 3
0 c 0
0 b 0
1
2 3
0 b 0
r2
20
20
a 30
30
10
0 c 0
r1
equal(1, 10)
return (
TRUE &&
TRUE &&
equal(3, 30)
return (
TRUE &&
TRUE &&
TRUE );
equal(1, 10)
return (
TRUE &&
TRUE &&
TRUE);
2 a 3
0 c 0
0 b 0
1
2 3
0 b 0
r2
20
20
a 30
30
10
0 c 0
r1
TRUE

BINARY SEARCH TREES
Dictionary
“A Dictionary is a collection of pairs, each pair has a key and an
associated item.”
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 55
ADT Dictionary is
objects: a collection of n > 0 pairs, each pair has a key and an
associated item.
functions:
for all d є Dictionary, item є Item, k є Key, n є integer
Dictionary Create(max_size) ::= create an empty dictionary.
Boolean IsEmpty(d, n) ::= if ( n > 0 ) return TRUE
else return FALSE
Element Search (d, k) ::= return item with key k,
return NULL if no such element.
Element Delete (d, k) ::= delete and return item (if any) with key
k.
void Insert (d, item, k) ::= insert item with key k into d.

BINARY SEARCH TREES
Definition
“A Binary Search Tree (BST) is a Binary tree. It may be empty. If
it is not empty then it satisfies the following properties:
(1) Each node has exactly one key and the keys in the tree are
distinct.
(2) The keys (if any) in the left subtree are smaller than the key
in the root.
(3) The keys (if any) in the right subtree are larger than the key
in the root.
(4) The left and right subtrees are also binary search trees.”
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 56
20
15
13 17
25
30
20
15
12 10
25
22
30
5
2
40

BINARY SEARCH TREES
Traversing a Binary Search Tree
 Traversing a BST is same as traversing a Binary tree.
 The different traversals are Inorder, Preorder and Postorder.
 Inorder traversal of a BST always yields all the nodes in
increasing order.
 To construct a Binary tree, we needed a combination of two
traversals, i.e., Inorder & Preorder or Inorder & Postorder.
 For a BST construction, we just need one of Preorder or
Postorder sequence.
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 57

BINARY SEARCH TREES
Example
Construct a BST using the Preorder traversal sequence below:
30 , 20 , 10 , 15 , 25 , 23 , 39 , 35 , 42
Solution
 We know that, the inorder traversal of a BST gives us the
elements of the tree in ascending order.
 In the problem statement, we have been given the preorder
sequence.
 Arranging the above elements in non decreasing order gives
us the inorder sequence for the BST, i.e,
10, 15, 20, 23, 25, 30, 35, 39, 42
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 58

BINARY SEARCH TREES
We have,
Preorder Sequence: 30, 20, 10, 15, 25, 23, 39, 35, 42
Inorder Sequence: 10, 15, 20, 23, 25, 30, 35, 39, 42
The resulting BST is,
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 59
30
10, 15, 20, 23, 25 35, 39, 42
30
10, 15
35, 39, 42
20
23, 25
30
10
35, 39, 42
20
23, 25
15
30
10
35, 39, 42
20
25
15 23
30
10
39
20
25
15 23
35 42

BINARY SEARCH TREES
Searching a Binary Search Tree
 Searching for a specific node in BST is pretty easy due to the fact
that, elements in BST are stored in a particular order.
Procedure
Step 1 - Compare the element with the root of the tree.
Step 2 - If the item is matched then return the location of the node.
Step 3 - Otherwise check if item is less than the element present on
root, if so then move to the left sub-tree.
Step 4 - If not, then move to the right sub-tree.
 Repeat this procedure recursively until match found.
 If element is not found then return NULL.
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 60

BINARY SEARCH TREES
Recursive Search
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 61
2 30 5
3 20 4 6 35 7
0 10 0 0 25 0 0 32 0 0 37 0
1
2 5
3 4 6 7
root
Key = 25
Search(1, 25)
Search(1, 25)
Search(2 , 25)
2 30 5
3 20 4 6 35 7
0 10 0 0 25 0 0 32 0 0 37 0
1
2 5
3 4 6 7
root
Search(1, 25)
Search(2 , 25)
Search(4, 25)
2 30 5
3 20 4 6 35 7
0 10 0 0 25 0 0 32 0 0 37 0
1
2 5
3 4 6 7
root
Search(1, 25)
Search(2 , 25)
TRUE
2 30 5
3 20 4 6 35 7
0 10 0 0 25 0 0 32 0 0 37 0
1
2 5
3 4 6 7
root
Search(1, 25)
TRUE
2 30 5
3 20 4 6 35 7
0 10 0 0 25 0 0 32 0 0 37 0
1
2 5
3 4 6 7
root
TRUE

BINARY SEARCH TREES
Iterative
Search
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 62
2 30 5
3 20 4 6 35 7
0 10 0 0 25 0 0 32 0 0 37 0
1
2 5
3 4 6 7
root
Key = 25
2 30 5
3 20 4 6 35 7
0 10 0 0 25 0 0 32 0 0 37 0
1
2 5
3 4 6 7
root
2 30 5
3 20 4 6 35 7
0 10 0 0 25 0 0 32 0 0 37 0
1
2 5
3 4 6 7
root
TRUE

BINARY SEARCH TREES
Insertion into a
BST
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 63
2 30 5
3 20 4 6 35 7
0 10 0 0 25 0 0 32 0 0 37 0
1
2 5
3 4 6 7
root
0 36 0
10
ptr
nodeptr
parentptr
2 30 5
3 20 4 6 35 7
0 10 0 0 25 0 0 32 0 0 37 0
1
2 5
3 4 6 7
root
0 36 0
10
ptr
nodeptr
parentptr
2 30 5
3 20 4 6 35 7
0 10 0 0 25 0 0 32 0 0 37 0
1
2 5
3 4 6 7
root
0 36 0
10
ptr
nodeptr
parentptr
2 30 5
3 20 4 6 35 7
0 10 0 0 25 0 0 32 0 0 37 0
1
2 5
3 4 6 7
root
0 36 0
10
ptr
nodeptr
parentptr
2 30 5
3 20 4 6 35 7
0 10 0 0 25 0 0 32 0 0 37 0
1
2 5
3 4 6 7
root
0 36 0
10
ptr
nodeptr
parentptr
2 30 5
3 20 4 6 35 7
0 10 0 0 25 0 0 32 0 0 37 0
1
2 5
3 4 6 7
root
0 36 0
10
ptr
parentptr
2 30 5
3 20 4 6 35 7
0 10 0 0 25 0 0 32 0 10 37 0
1
2 5
3 4 6 7
root
0 36 0
10
parentptr

BINARY SEARCH TREES
Deletion from a BST
 This operation involves deletion of a specified node from a
binary search tree.
 However, we must delete a node in such a way that the
property of BST doesn't violate.
 There are three situations of deleting a node from binary
search tree.
 The node to be deleted is a leaf node.
 The node to be deleted has only one child.
 The node to be deleted has two children.
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 64

BINARY SEARCH TREES
Case 1 - The node to be deleted is a leaf node
 It is the simplest case.
 In this case, replace the leaf node with the NULL and simple
free the allocated space.
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 65
30
10
39
20
25 35 42
Delete
42
30
10
39
20
25 35 42
30
10
39
20
25 35 NULL

BINARY SEARCH TREES
Case 2 - The node to be deleted has only one child
 In this case, first replace the node with its child.
 Then delete this child node.
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 66
30
10
39
20
25 42
Delete
39
30
10
42
20
25 39
30
10
42
20
25

BINARY SEARCH TREES
Case 3 - The node to be deleted has two children
 This is a case that is a bit complex compared to the previous
cases.
 To execute this case, we need to first understand the concept
of Inorder Successor.
 In a BST, Inorder Successor of a node is the next node in the
Inorder traversal of the tree.
 Inorder Successor is NULL for the last node in the tree.
 We now find the inorder successor of various nodes in a BST.
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 67

BINARY SEARCH TREES
 The inorder traversal of the above tree is
10, 15, 20, 23, 25, 30, 35, 39, 42
 Here, the inorder successors of node 10, 20, 30 and 35 are,
15, 23, 35, 39
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 68
30
10
39
20
25
15 23
35 42

BINARY SEARCH TREES
There are 2 ways to find the Inorder successor of a node in a
BST:
Node with a Right Subtree
 If a node has a right subtree, its Inorder successor is the node
with the least value in its right subtree.
 The leftmost node in the right subtree is the node that will have
the least value.
Node without a Right Subtree
 If the right subtree doesn’t exist for a node, then the Inorder
successor is one of its ancestors.
 For a node, its ancestor with the next big value is its Inorder
successor.
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 69

BINARY SEARCH TREES
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 70
30
10
39
20
25
15 23
35 42
5
21
27 40 50
The Inorder Successor of 20 is
30
10
39
20
25
15 23
35 42
5
21
27 40 50
Right
Subtree
30
10
39
20
25
15 23
35 42
5
21
27 40 50
Leftmost
Node in Right
Subtree
The Inorder Successor of 20 is
21
The Inorder Successor of 5 is
30
10
39
20
25
15 23
35 42
5
21
27 40 50
Ancestor
s
30
10
39
20
15 23
35 42
5
21
27 40 50
Ancestor with
the next big
value
25
The Inorder Successor of 5 is
10
30
10
39
20
15 23
35 42
5
21
27 40 50
25
The Inorder Successor of 39 is
The Inorder Successor of 39 is
40
The Inorder Successor of 15 is
The Inorder Successor of 15 is
20
The Inorder Successor of 50 is
The Inorder Successor of 50 is
NULL

BINARY SEARCH TREES
 Now that we are familiar with the concept of Inorder Successor, lets
us discuss how deletion takes place in a BST when a node has both
children.
 If a node to be deleted has both its children, then we swap this node
with its Inorder Successor.
 The node is then deleted.
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 71
30
10
39
20
15 23
35 42
5 27 40 50
25
Delete 30
30
10
39
20
15 23
35 42
5 27 40 50
25
35
10
39
20
15 23
30 42
5 27 40 50
25
35
10
39
20
15 23
42
5 27 40 50
25
35
10
39
20
15 23
42
5 27 40 50
25
Delete 20
35
10
39
23
15 20
42
5 27 40 50
25
35
10
39
23
15
42
5 27 40 50
25

Dr. K. Balakrishnan, 18CS32, Dept. of
APPLICATION OF TREES – EVALUATION OF
EXPRESSIONS
Expression Tree
 As the name suggests Expression Tree is nothing but
expressions arranged in a tree-like data structure.
 In an expression tree the leaf nodes have the values to be
operated.
 The internal nodes contain the operator using which the leaf
nodes will be operated.
3/27/2023 72
+
4 *
+ 2
7 9

APPLICATION OF TREES – EVALUATION OF
EXPRESSIONS
Expression Tree Construction
 The most popular way of constructing an expression tree is by
using the Postfix Expression.
 Also for the construction, we use the Stack data structure.
Procedure
Scan through the input postfix expression and do the following for
every character.
1. If the scanned character is an operand push it into the stack.
2. If the scanned character is an operator, pop top two values
from the stack, make them the right and left child of the
operator and push the result back onto the stack.
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 73

APPLICATION OF TREES – EVALUATION OF
EXPRESSIONS
Example
Construct the expression tree for the following postfix expression
3 5 9 + 2 * +
Solution
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 74
Scanned Symbol = 3
Stack
3
Scanned Symbol = 5
5
3
Scanned Symbol = 9
9
5
3
Scanned Symbol = +
5
3
+
9
3 5
Scanned Symbol = 2
2
3
Scanned Symbol = *
*
3
2
+
9
5
*
2
Scanned Symbol = + +
Expression Tree
+
9
5
*
2
+
3

APPLICATION OF TREES – EVALUATION OF
EXPRESSIONS
Practice Examples
Construct the expression tree for the given postfix expressions.
1. A B + C D - *
2. A B C / - A K / L - *
3. A B + C D E + * *
 So far we have seen how an expression tree can be
constructed from a given postfix expression.
 Sometimes, the input data for expression tree ocnstruction will
be an infix or prefix expression.
 In such cases, we need to first covert the infix or prefix
expression into postfix expression and then construct the
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 75

APPLICATION OF TREES – EVALUATION OF
EXPRESSIONS
 We have already discussed the shortcut method to convert an
Infix expression to Postfix expression.
 So we now only focus on Prefix to Postfix conversion.
Procedure
1. Read the Prefix expression in reverse order (from right to left).
2. If the symbol is an operand, then push it onto the Stack.
3. If the symbol is an operator, then pop two operands from the
Stack.
4. Create a string by concatenating the two operands and the
operator after them.
string = operand1 + operand2 + operator
Push the resultant string back to Stack.
5. Repeat the above steps until end of Prefix expression.
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 76

APPLICATION OF TREES – EVALUATION OF
EXPRESSIONS
Example
Convert the expression * + A B – C D into its postfix form.
Solution
Prefix Expression : * + A B – C D
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 77
Symbol Scanned :
D
D
Stack
Symbol Scanned :
C
C
D
Symbol Scanned : -
Operand 1 : C
D
Operand 1 : C
Operand 2 : D
Operand 1 : C
Operand 2 : D
String : C D -
C D -
Symbol Scanned :
B
B
C D -
Symbol Scanned :
A
A
B
C D -
Symbol Scanned : +
Operand 1 : A
Operand 2 : B
String : A B +
C D -
A B +
C D -
Symbol Scanned : *
Operand 1 : A B +
Operand 2 : C D -
Postfix : A B + C D
- *

APPLICATION OF TREES – EVALUATION OF
EXPRESSIONS
Practice Examples
Construct the expression tree for the following expressions:
1. a + (b * c) + d * (e + f)
2. 7 + ( ( 1 + 8 ) * 3 )
3. (5-x)*y+6/(x + z)
4. + + A * B C D
5. + * A B * C D
6. + + + A B C D
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 78

THREADED BINARY TREES
Consider the binary tree below.
There are some disadvantages associated with the above tree:
 Only downward movement is allowed in the above binary
tree.
d
a
b c
e f g
h i
0 0 0 0
0 0 0 0 0 0
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 79

THREADED BINARY TREES
 These drawbacks are overcome by Threaded Binary Trees.
 In a Threaded Binary Tree, the NULL values in the link fields of
the nodes are replaced by pointers to some other nodes in the
tree.
 These special pointers that replace the NULL values are
referred to as Threads. Hence the name Threaded Binary
Trees.
 In order to replace the NULL links with pointers to other nodes,
we follow the below mentioned rules:
 If the left child/link of a node is NULL, then this NULL value is replaced
by a pointer to the Inorder Predecessor of the node.
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 80

THREADED BINARY TREES
 We will now understand the terms Inorder Predecessor and
Inorder Successor using the following binary tree as example.
a
b c
d e f g
h i
0 0 0 0
0 0 0 0 0 0
 We first perform the Inorder Traversal for the above tree. We
get,
h d i b e a f c g
 The Inorder Predecessor and Inorder Successor are then
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 81

THREADED BINARY TREES
 The Inorder Predecessor for a node X is the node that
precedes X in the inorder traversal.
 The Inorder Successor for a node X is the node that
succeeds X in the inorder traversal.
a
b c
d e f g
h i
0 0 0 0
0 0 0 0 0 0
 The Inorder Traversal for the above tree is,
h d i b e a f c g
h i
e f g
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 82

THREADED BINARY TREES
C implementation of Threaded Binary Trees
 Implementing a Threaded Binary tree is very similar to that of a
normal binary tree, with one addition.
 A node in a threaded binary tree may have a link or a
thread.
 It is necessary to differentiate the two as a thread and a link
are very different connections.
 Hence, the structure definition to represent a node of a
threaded binary tree will have two additional members namely,
leftThread and rightThread to indicate the presence of a
thread.
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 83

THREADED BINARY TREES
 The structure definition for a threaded binary tree is
struct node{
bool leftThread;
struct node *leftChild;
char data;
struct node *rightChild;
bool rightThread;
};
Typedef struct node threadTree;
leftThread leftChild data rightChild
rightThrea
d
threadTree
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 84

THREADED BINARY TREES
 A threaded binary tree and its memory representation is
presented below,
a
b c
d e f g
F a F
F b F F c F
T d T T e T T f T T g T
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 85

THREADED BINARY TREES
 There is one issue with the threaded binary tree that has been
presented.
 We see that the leftThread of node d and rightThread of node
g are pointing to nowhere.
 This is a case of dangling pointers and dangling pointers are
dangerous.
 Hence, we need to make sure that no threads in a threaded
binary tree are dangling pointers.
 For this, we introduce a Header Node in a threaded binary
tree.
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 86

THREADED BINARY TREES
 All threads that are pointing to nowhere are hereafter made to
point to the header node.
 This header node is the first node of the threaded binary
tree.
 The root pointer, which usually points to the root node of the
tree will now point to the header node.
 The actual threaded binary tree becomes the left subtree of the
header node.
 The threaded binary tree with the header node is presented
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 87

THREADED BINARY TREES
F a F
F b F F c F
T d T T e T T f T T g T
F F
ROO
T
Header Node
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 88

END OF
MODULE 4
3/27/2023 Dr. K. Balakrishnan, 18CS32, Dept. of CSE, 89