binary_tree.py

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

    def __str__(self):
        """Returns a string representation of the tree using Preorder Traversal."""
        result = []

        def preorder(node):
            if not node:
                return
            result.append(str(node.val))
            preorder(node.left)
            preorder(node.right)

        preorder(self)
        return " -> ".join(result)


from collections import deque


def create_binary_tree(values):
    """
    Creates a binary tree from a level-order list.
    Example: [1, 2, 3, None, 4, 5] → Binary Tree
        1
       / \
      2   3
       \  /
        4 5
    """
    if not values:
        return None

    root = TreeNode(values[0])
    queue = deque([root])
    i = 1

    while i < len(values):
        curr = queue.popleft()

        if i < len(values) and values[i] is not None:
            curr.left = TreeNode(values[i])
            queue.append(curr.left)
        i += 1

        if i < len(values) and values[i] is not None:
            curr.right = TreeNode(values[i])
            queue.append(curr.right)
        i += 1

    return root