Size of a tree is the number of elements present in the tree. Size of the below tree is 5.

Size() function recursively calculates the size of a tree. It works as follows:

Size of a tree = Size of left subtree + 1 + Size of right subtree.

**Algorithm:**

size(tree) 1. If tree is empty then return 0 2. Else (a) Get the size of left subtree recursively i.e., call size( tree->left-subtree) (a) Get the size of right subtree recursively i.e., call size( tree->right-subtree) (c) Calculate size of the tree as following: tree_size = size(left-subtree) + size(right- subtree) + 1 (d) Return tree_size

## C

#include <stdio.h> #include <stdlib.h> /* A binary tree node has data, pointer to left child and a pointer to right child */ struct node { int data; struct node* left; struct node* right; }; /* Helper function that allocates a new node with the given data and NULL left and right pointers. */ struct node* newNode(int data) { struct node* node = (struct node*) malloc(sizeof(struct node)); node->data = data; node->left = NULL; node->right = NULL; return(node); } /* Computes the number of nodes in a tree. */ int size(struct node* node) { if (node==NULL) return 0; else return(size(node->left) + 1 + size(node->right)); } /* Driver program to test size function*/ int main() { struct node *root = newNode(1); root->left = newNode(2); root->right = newNode(3); root->left->left = newNode(4); root->left->right = newNode(5); printf("Size of the tree is %d", size(root)); getchar(); return 0; }

## Java

// A recursive Java program to calculate the size of the tree /* Class containing left and right child of current node and key value*/ class Node { int data; Node left, right; public Node(int item) { data = item; left = right = null; } } /* Class to find size of Binary Tree */ class BinaryTree { Node root; /* Given a binary tree. Print its nodes in level order using array for implementing queue */ int size() { return size(root); } /* computes number of nodes in tree */ int size(Node node) { if (node == null) return 0; else return(size(node.left) + 1 + size(node.right)); } public static void main(String args[]) { /* creating a binary tree and entering the nodes */ BinaryTree tree = new BinaryTree(); tree.root = new Node(1); tree.root.left = new Node(2); tree.root.right = new Node(3); tree.root.left.left = new Node(4); tree.root.left.right = new Node(5); System.out.println("The size of binary tree is : " + tree.size()); } }

## Python

# Python Program to find the size of binary tree # A binary tree node class Node: # Constructor to create a new node def __init__(self, data): self.data = data self.left = None self.right = None # Computes the number of nodes in tree def size(node): if node is None: return 0 else: return (size(node.left)+ 1 + size(node.right)) # Driver program to test above function root = Node(1) root.left = Node(2) root.right = Node(3) root.left.left = Node(4) root.left.right = Node(5) print "Size of the tree is %d" %(size(root)) # This code is contributed by Nikhil Kumar Singh(nickzuck_007)

Output:

Size of the tree is 5

**Time & Space Complexities:** Since this program is similar to traversal of tree, time and space complexities will be same as Tree traversal (Please see our Tree Traversal post for details)