bt_preorder.py

from pathlib import Path
import sys

sys.path.append(str(Path(__file__).resolve().parent.parent))

from binary_tree import create_binary_tree


class Solution:
    def preorderTraversal(self, root):
        if root is None:
            return []

        result = []

        def preorder(node):
            if node is None:
                return []

            result.append(node.val)
            preorder(node.left)
            preorder(node.right)

        preorder(root)
        return result


if __name__ == "__main__":
    solution = Solution()

    root = [1, None, 2, 3]
    bt = create_binary_tree(root)
    result = solution.preorderTraversal(bt)
    assert result == [1, 2, 3]
    print("Test Case 1 Passed!")
    print(result)