Study notes for Trees

By BYJU'S Exam Prep

Updated on: September 25th, 2023

A tree is a non-linear and hierarchical Data Structure.

Trees represent data containing a hierarchical relationship between elements e. g., records, family trees and table contents. A tree is a data structure based on a hierarchical tree structure with a set of nodes.



Study notes for Trees

  • Trees can be used

    • for underlying structure in decision-making algorithms
    • to represent Heaps (Priority Queues)
    • to represent B-Trees (fast access to the database)
    • for storing hierarchies in organizations
    • for file system

    Binary Tree 

    A binary tree is a tree-like structure that is rooted and in which each node has at most two children, and each child of a node is designated as its left or right child. In this kind of tree, the maximum degree of any node is at most 2. A binary tree T is defined as a finite set of elements such that

    • T is empty (called NULL tree or empty tree).
    • T contains a distinguished Node R called the root of T, and the remaining nodes of T form an ordered pair of disjoint binary trees T1 and T2.
  • Node: Each data item in a tree.
  • Root: First or top data item in a hierarchical arrangement.
  • Degree of a Node: Number of subtrees of a given node.Example: Degree of A = 3, Degree of E = 2
  • Degree of a Tree: Maximum degree of a node in a tree.Example: Degree of above tree = 3
  • Depth or Height: Maximum level number of a node + 1(i.e., level number of a farthest leaf node of a tree + 1).Example: Depth of above tree = 3 + 1= 4
  • Non-terminal Node: Any node except root node whose degree is not zero.
  • Forest: Set of disjoint trees.
  • Siblings: D and G are siblings of parent Node B.
  • Path: Sequence of consecutive edges from the source node to the destination node. 
  • Internal nodes: All nodes that have children nodes are called internal nodes.
  • Leaf nodes: Those nodes, which have no child, are called leaf nodes. 
  • The depth of a node is the number of edges from the root to the node. 
  • The height of a node is the number of edges from the node to the deepest leaf. 
  • The height of a tree is the height of the root.

Study notes for Trees

Any node N in a binary tree T has either 0, 1 or 2 successors. Level l of a binary tree T can have at most 2l nodes. The number of nodes on each level i of binary tree is at most 2i

  • The number n of nodes in a binary tree of height h is at least n = h + 1 and at most n = 2h+1 – 1, where h is the depth of the tree.
  • Depth d of a binary tree with n nodes >= floor(log n)
    • d = floor(log N); lower bound, when a tree is a full binary tree
    • d = n – 1; upper bound, when a tree is a degenerate tree

Creation of a binary tree

void insert(node ** tree, int val) {
 node *temp = NULL;
 if(!(*tree)) {
   temp = (node *)malloc(sizeof(node));
   temp->left = temp->right = NULL;
   temp->data = val;
   *tree = temp;

 if(val < (*tree)->data) {
      insert(&(*tree)->left, val);
 else if(val > (*tree)->data) {
     insert(&(*tree)->right, val);

Search an element into binary tree

node* search(node ** tree, int val) {
 if(!(*tree)) {
   return NULL;
 if(val == (*tree)->data) {
   return *tree;
 else if(val < (*tree)->data) {
    search(&((*tree)->left), val);
 else if(val > (*tree)->data){
    search(&((*tree)->right), val);

Delete an element from binary tree

void deltree(node * tree) {
 if (tree) {

Strictly Binary Trees

If every non-terminal node in a binary tree consists of non-empty left subtree and right subtree. In other words, if any node of a binary tree has either 0 or 2 child nodes, then such tree is known as strictly binary tree or extended binary tree or 2- tree. Complete Binary Tree: A complete binary tree is a tree in which every level, except possibly the last, is filled. A complete binary tree has the following properties

  • Which can have 0, 1 or 2 children.
  • First, we need to fill the left node, then the right node in a level.
  • We can start putting data items in the next level only when the previous level is filled.
  • A complete binary tree of the height h has between 2h and 2(h+1)-1 nodes.

Tree Traversal: Three types of tree traversal are given below 


    • Process the root R.
    • Traverse the left subtree of R in preorder.
    • Traverse the right subtree of R in preorder.

/* Recursive function to print the elements of a binary tree with preorder traversal*/
void preorder(struct btreenode *node)
  if (node != NULL)
    printf(\%d, node->data);


  • Traverse the left subtree of R in inorder.
  • Process the root R.
  • Traverse the right subtree of R in inorder.
/* Recursive function to print the elements of a binary tree with inorder traversal*/
void inorder(struct btreenode *node)
  if (node != NULL)
    printf(\%d, node->data);


  • Traverse the left subtree of R in postorder.
  • Traverse the right subtree of R in postorder.
  • Process the root R.
/* Recursive function to print the elements of a binary tree with postorder traversal*/
void postorder(struct btreenode *node)
  if (node != NULL)
    printf(\%d, node->data);

Breadth First Traversal (BFT): The breadth-first traversal of a tree visits the nodes in the order of their depth in the tree. BFT first visits all the nodes at depth zero (i.e., root), then all the nodes at depth 1 and so on. At each depth, the nodes are visited from left to right.

Study notes for Trees
Depth First Traversal (DFT): In DFT, one starts from the root and explores as far as possible along each branch before backtracking.
Study notes for Trees
Perfect Binary Tree or Full Binary Tree: A binary tree in which all leaves are at the same level or the same depth and in which every parent has two children.
Study notes for Trees

All leaves (D, E, F, G) are at depth 3 or 2, and every parent has exactly 2 children. Let a binary tree contain MAX, the maximum number of nodes possible for its height h. Then h= log(MAX + 1) –1.

The height of the Binary Search Tree equals the number of links of the path from the root node to the deepest node.

    • Number of internal/leaf nodes in a full binary tree of height h2h leaves
    • 2h -1 internal nodes

Expression Tree

An expression tree is a binary tree that represents a binary arithmetic expression. All internal nodes in the expression tree are operators, and leaf nodes are the operands. The expression tree will help in the precedence relation of operators. (2+3)*4 and 2+(3*4) expressions will have different expression trees.


Example1: Recursive function for size (number of nodes) of a binary tree

int size(struct btreenode *node)
  if (node == NULL)
    return 0;
    return (1 + size(node->left) + size(node->right));

Example2: Recursive function for Height of a tree

(Hieght is the length of path to the deepest node from the root node of tree)

int height(struct btreenode *node)
if (node == NULL) return 0;
else return (1 + Max(height(node->left), height(node->right)));

Example3: Print the elements of binary tree using level order traversal

void levelorder(struct node* root)
int rear, front;
struct node **queue = createqueue(&front, &rear);
struct node *tempnode = root;
while (temp_node)
printf(\%d , tempnode->data);
if (tempnode->left)
enqueue(queue, &rear, tempnode->left);
if (tempnode->right)
enqueue(queue, &rear, tempnode->right);
tempnode = dequeue(queue, &front);
struct node** createqueue(int *front, int *rear)
struct node **queue = (struct node **) malloc(sizeof(struct node*)*n);
*front = *rear = 0;
return queue;

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