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Binary Tree in Python

Last Updated : 23 Jul, 2025

Binary Tree is a non-linear and hierarchical data structure where each node has at most two children referred to as the left child and the right child. The topmost node in a binary tree is called the root, and the bottom-most nodes are called leaves.

Introduction-to-Binary-Tree — Introduction to Binary Tree

Representation of Binary Tree

Each node in a Binary Tree has three parts:

Data
Pointer to the left child
Pointer to the right child

Binary-Tree-Representation- — Binary Tree Representation

Create/Declare a Node of a Binary Tree in Python

Syntax to declare a Node of Binary Tree in Python:

Python

# A Python class that represents
# an individual node in a Binary Tree
class Node:
    def __init__(self, key):
        self.left = None
        self.right = None
        self.val = key

Example for Creating a Binary Tree in Python

Here’s an example of creating a Binary Tree with four nodes (2, 3, 4, 5)

Binary-Tree-with-three-nodes- — Creating a Binary Tree having three nodes

Python

class Node:
    def __init__(self, d):
        self.data = d
        self.left = None
        self.right = None

# Initialize and allocate memory for tree nodes
firstNode = Node(2)
secondNode = Node(3)
thirdNode = Node(4)
fourthNode = Node(5)

# Connect binary tree nodes
firstNode.left = secondNode
firstNode.right = thirdNode
secondNode.left = fourthNode

In the above code, we have created four tree nodes firstNode, secondNode, thirdNode and fourthNode having values 2, 3, 4 and 5 respectively.

After creating three nodes, we have connected these node to form the tree structure like mentioned in above image.
link secondNode to the left child of firstNode by firstNode.left = secondNode.
link thirdNode to the right child of firstNode by firstNode.right = thirdNode.
link fourthNode to the left child of secondNode by secondNode.left = fourthNode.

Types of Binary Tree

Binary Tree can be classified into multiples types based on multiple factors:

On the basis of Number of Children
On the basis of Completion of Levels
On the basis of Node Values:

read more about - Terminologies and properties of Binary Tree

Operations On Binary Tree

Following is a list of common operations that can be performed on a binary tree:

1. Traversal in Binary Tree

Traversal in Binary Tree involves visiting all the nodes of the binary tree. Tree Traversal algorithms can be classified broadly into two categories, DFS and BFS:

Depth-First Search (DFS) algorithms: DFS explores as far down a branch as possible before backtracking. It is implemented using recursion. The main traversal methods in DFS for binary trees are:

Preorder Traversal (current-left-right): Visits the node first, then left subtree, then right subtree.
Inorder Traversal (left-current-right): Visits left subtree, then the node, then the right subtree.
Postorder Traversal (left-right-current): Visits left subtree, then right subtree, then the node.

Breadth-First Search (BFS) algorithms: BFS explores all nodes at the present depth before moving on to nodes at the next depth level. It is typically implemented using a queue. BFS in a binary tree is commonly referred to as Level Order Traversal.

Below is the implementation of traversals algorithm in binary tree:

Python

class Node:
    def __init__(self, data):
        self.data = data
        self.left = None
        self.right = None

# In-order DFS: Left, Root, Right
def in_order_dfs(node):
    if node is None:
        return
    in_order_dfs(node.left)
    print(node.data, end=' ')
    in_order_dfs(node.right)

# Pre-order DFS: Root, Left, Right
def pre_order_dfs(node):
    if node is None:
        return
    print(node.data, end=' ')
    pre_order_dfs(node.left)
    pre_order_dfs(node.right)

# Post-order DFS: Left, Right, Root
def post_order_dfs(node):
    if node is None:
        return
    post_order_dfs(node.left)
    post_order_dfs(node.right)
    print(node.data, end=' ')

# BFS: Level order traversal
def bfs(root):
    if root is None:
        return
    queue = [root]
    while queue:
        node = queue.pop(0)
        print(node.data, end=' ')
        if node.left:
            queue.append(node.left)
        if node.right:
            queue.append(node.right)

if __name__ == "__main__":
    # Creating the tree
    root = Node(2)
    root.left = Node(3)
    root.right = Node(4)
    root.left.left = Node(5)

    print("In-order DFS: ", end='')
    in_order_dfs(root)
    print("\nPre-order DFS: ", end='')
    pre_order_dfs(root)
    print("\nPost-order DFS: ", end='')
    post_order_dfs(root)
    print("\nLevel order: ", end='')
    bfs(root)

Output

In-order DFS: 5 3 2 4 
Pre-order DFS: 2 3 5 4 
Post-order DFS: 5 3 4 2 
Level order: 2 3 4 5

2. Insertion in Binary Tree

Inserting elements means add a new node into the binary tree. As we know that there is no such ordering of elements in the binary tree, So we do not have to worry about the ordering of node in the binary tree. We would first creates a root node in case of empty tree. Then subsequent insertions involve iteratively searching for an empty place at each level of the tree. When an empty left or right child is found then new node is inserted there. By convention, insertion always starts with the left child node.

Python

from collections import deque

class Node:
    def __init__(self, d):
        self.data = d
        self.left = None
        self.right = None

# Function to insert a new node in the binary tree
def insert(root, key):
    if root is None:
        return Node(key)

    # Create a queue for level order traversal
    queue = deque([root])

    while queue:
        temp = queue.popleft()

        # If left child is empty, insert the new node here
        if temp.left is None:
            temp.left = Node(key)
            break
        else:
            queue.append(temp.left)

        # If right child is empty, insert the new node here
        if temp.right is None:
            temp.right = Node(key)
            break
        else:
            queue.append(temp.right)

    return root

# In-order traversal
def inorder(root):
    if root is None:
        return
    inorder(root.left)
    print(root.data, end=" ")
    inorder(root.right)

if __name__ == "__main__":
    root = Node(2)
    root.left = Node(3)
    root.right = Node(4)
    root.left.left = Node(5)

    print("Inorder traversal before insertion: ", end="")
    inorder(root)
    print()

    key = 6
    root = insert(root, key)

    print("Inorder traversal after insertion: ", end="")
    inorder(root)
    print()

Output

Inorder traversal before insertion: 5 3 2 4 
Inorder traversal after insertion: 5 3 6 2 4

3. Searching in Binary Tree

Searching for a value in a binary tree means looking through the tree to find a node that has that value. Since binary trees do not have a specific order like binary search trees, we typically use any traversal method to search. The most common methods are depth-first search (DFS) and breadth-first search (BFS). In DFS, we start from the root and explore the depth nodes first. In BFS, we explore all the nodes at the present depth level before moving on to the nodes at the next level. We continue this process until we either find the node with the desired value or reach the end of the tree. If the tree is empty or the value isn’t found after exploring all possibilities, we conclude that the value does not exist in the tree.

Here is the implementation of searching in a binary tree using Depth-First Search (DFS):

Python

class Node:
    def __init__(self, d):
        self.data = d
        self.left = None
        self.right = None

# Function to search for a value in the binary tree using DFS
def searchDFS(root, value):
    # Base case: If the tree is empty or we've reached a leaf node
    if root is None:
        return False
    # If the node's data is equal to the value we are searching for
    if root.data == value:
        return True
    # Recursively search in the left and right subtrees
    left_res = searchDFS(root.left, value)
    right_res = searchDFS(root.right, value)

    return left_res or right_res

if __name__ == "__main__":
    root = Node(2)
    root.left = Node(3)
    root.right = Node(4)
    root.left.left = Node(5)
    root.left.right = Node(6)

    value = 6
    if searchDFS(root, value):
        print(f"{value} is found in the binary tree")
    else:
        print(f"{value} is not found in the binary tree")

Output

6 is found in the binary tree

4. Deletion in Binary Tree

Deleting a node from a binary tree means removing a specific node while keeping the tree’s structure. First, we need to find the node that want to delete by traversing through the tree using any traversal method. Then replace the node’s value with the value of the last node in the tree (found by traversing to the rightmost leaf), and then delete that last node. This way, the tree structure won’t be effected. And remember to check for special cases, like trying to delete from an empty tree, to avoid any issues.

Note: There is no specific rule of deletion but we always make sure that during deletion the binary tree proper should be preserved.

Deletion-in-Binary-Tree — Deletion in Binary Tree

Python

from collections import deque

class Node:
    def __init__(self, d):
        self.data = d
        self.left = None
        self.right = None

# Function to delete a node from the binary tree
def deleteNode(root, val):
    if root is None:
        return None

    # Use a queue to perform BFS
    queue = deque([root])
    target = None

    # Find the target node
    while queue:
        curr = queue.popleft()

        if curr.data == val:
            target = curr
            break
        if curr.left:
            queue.append(curr.left)
        if curr.right:
            queue.append(curr.right)

    if target is None:
        return root

    # Find the deepest rightmost node and its parent
    last_node = None
    last_parent = None
    queue = deque([(root, None)])

    while queue:
        curr, parent = queue.popleft()
        last_node = curr
        last_parent = parent

        if curr.left:
            queue.append((curr.left, curr))
        if curr.right:
            queue.append((curr.right, curr))

    # Replace target's value with the last node's value
    target.data = last_node.data

    # Remove the last node
    if last_parent:
        if last_parent.left == last_node:
            last_parent.left = None
        else:
            last_parent.right = None
    else:
        return None
    return root

# In-order traversal
def inorder(root):
    if root is None:
        return
    inorder(root.left)
    print(root.data, end=" ")
    inorder(root.right)

if __name__ == "__main__":
    root = Node(2)
    root.left = Node(3)
    root.right = Node(4)
    root.left.left = Node(5)
    root.left.right = Node(6)

    print("Original tree (in-order): ", end="")
    inorder(root)
    print()

    val_to_del = 3
    root = deleteNode(root, val_to_del)

    print(f"Tree after deleting {val_to_del} (in-order): ", end="")
    inorder(root)
    print()

Output

Original tree (in-order): 5 3 6 2 4 
Tree after deleting 3 (in-order): 5 6 2 4

Auxiliary Operations On Binary Tree

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