Python Program to Print only Nodes in Left SubTree

When it is required to print the nodes in the left subtree, a binary tree class can be created with methods to perform operations like setting the root node, inserting elements, and traversing the tree. This program demonstrates how to build a binary tree and display only the nodes in the left subtree.

A binary tree is a hierarchical data structure where each node has at most two children: a left child and a right child. The left subtree contains all nodes that are descendants of the left child of the root node.

Binary Tree Implementation

class BinaryTree:
    def __init__(self, data=None):
        self.key = data
        self.left = None
        self.right = None
    
    def set_root(self, data):
        self.key = data
    
    def inorder_traversal(self):
        if self.left is not None:
            self.left.inorder_traversal()
        print(self.key, end=' ')
        if self.right is not None:
            self.right.inorder_traversal()
    
    def insert_at_left(self, new_node):
        self.left = new_node
    
    def insert_at_right(self, new_node):
        self.right = new_node
    
    def search_elem(self, key):
        if self.key == key:
            return self
        if self.left is not None:
            temp = self.left.search_elem(key)
            if temp is not None:
                return temp
        if self.right is not None:
            temp = self.right.search_elem(key)
            return temp
        return None
    
    def print_left_subtree(self):
        if self.left is not None:
            self.left.inorder_traversal()

# Create a binary tree
root = BinaryTree(10)
root.insert_at_left(BinaryTree(5))
root.insert_at_right(BinaryTree(15))
root.left.insert_at_left(BinaryTree(3))
root.left.insert_at_right(BinaryTree(7))

print("Left subtree nodes: ", end="")
root.print_left_subtree()
print()

The output of the above code is ?

Left subtree nodes: 3 5 7

Interactive Binary Tree Program

Here's a more comprehensive program that allows interactive tree building ?

class BinaryTree:
    def __init__(self, data=None):
        self.key = data
        self.left = None
        self.right = None
    
    def inorder_traversal(self):
        if self.left is not None:
            self.left.inorder_traversal()
        print(self.key, end=' ')
        if self.right is not None:
            self.right.inorder_traversal()
    
    def insert_at_left(self, new_node):
        self.left = new_node
    
    def insert_at_right(self, new_node):
        self.right = new_node
    
    def search_elem(self, key):
        if self.key == key:
            return self
        if self.left is not None:
            temp = self.left.search_elem(key)
            if temp is not None:
                return temp
        if self.right is not None:
            temp = self.right.search_elem(key)
            return temp
        return None
    
    def print_left_subtree(self):
        if self.left is not None:
            self.left.inorder_traversal()

my_tree = None

print('Menu (assumes no duplicate keys)')
print('insert <data> at root')
print('insert <data> left of <data>')
print('insert <data> right of <data>')
print('left')
print('quit')

while True:
    user_input = input('What operation would you like to perform? ').split()
    
    operation = user_input[0].strip().lower()
    
    if operation == 'insert':
        data = int(user_input[1])
        new_node = BinaryTree(data)
        suboperation = user_input[2].strip().lower()
        
        if suboperation == 'at':
            my_tree = new_node
        else:
            position = user_input[4].strip().lower()
            key = int(position)
            ref_node = None
            
            if my_tree is not None:
                ref_node = my_tree.search_elem(key)
            
            if ref_node is None:
                print('No such key found')
                continue
            
            if suboperation == 'left':
                ref_node.insert_at_left(new_node)
            elif suboperation == 'right':
                ref_node.insert_at_right(new_node)
    
    elif operation == 'left':
        print('Nodes in the left subtree: ', end='')
        if my_tree is not None:
            my_tree.print_left_subtree()
            print()
        else:
            print('Tree is empty')
    
    elif operation == 'quit':
        break

How It Works

The print_left_subtree() method works by checking if the root node has a left child. If it does, it performs an inorder traversal of the entire left subtree, which visits all nodes in the left subtree in the order: left ? root ? right.

In this tree structure, the left subtree (highlighted in orange) contains nodes 5, 3, and 7. When print_left_subtree() is called on the root, it traverses only these nodes.

Key Features

• BinaryTree Class: Contains the tree structure with left and right pointers
• Inorder Traversal: Visits nodes in left-root-right order
• Search Function: Finds a specific node by its key value
• Left Subtree Display: Shows only nodes in the left subtree

Conclusion

This program demonstrates how to build a binary tree and extract nodes from the left subtree only. The print_left_subtree() method performs inorder traversal on the left child, displaying all nodes in that subtree in sorted order.