Binary Tree Inorder Traversal in Python

Define a TreeNode class and recursively print in-order traversal (left, node, right) of a binary tree.

Easy Python 3.9+ Aug 9, 2026 OOP & classes 14 views 0 copies

binary-tree recursion traversal tree

Python code

24 lines

Python 3.9+

class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right


def inorder_traversal(root):
    return inorder_traversal(root.left) + [root.val] + inorder_traversal(root.right) if root else []


if __name__ == "__main__":
    # Build a sample tree:      1
    #                          / \
    #                         2   3
    #                        / \
    #                       4   5
    root = TreeNode(1)
    root.left = TreeNode(2)
    root.right = TreeNode(3)
    root.left.left = TreeNode(4)
    root.left.right = TreeNode(5)

    print(inorder_traversal(root))

Output

stdout

[4, 2, 5, 1, 3]

How it works

The TreeNode class models each node with a value and left/right children, defaulting to None. The inorder_traversal function recursively visits the left subtree, appends the current node's value, then visits the right subtree. The base case returns an empty list for a None node, ensuring clean concatenation. Recursion naturally follows the LNR (left-node-right) order required for in-order traversal.

Common mistakes

Forgetting the base case return [] when root is None, causing a TypeError.
Placing the node value append in the wrong position, breaking the LNR order.
Building the tree incorrectly by not linking left/right children properly.

Variations

Use an explicit stack for an iterative version to avoid recursion depth limits.
Collect nodes in a separate list parameter instead of concatenating lists.

Real-world use cases

Displaying binary search tree keys in sorted order for reporting tools.
Validating a binary search tree by checking in-order output is strictly increasing.
Converting a tree to a flat sorted list for exporting data to another system.

Binary Tree Inorder Traversal in Python

Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

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Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

More from OOP & classes

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