def sum_all(root):
    """Sums up all node values in the tree.
    This will fail if all node values are not numerical.
    Args:
        root(BinaryTreeNode): Root node
    Returns:
        int: Sum of all node values or 0 if empty tree.
    """
    if root is None:
        return 0
    return root.data + sum_all(root.lchild) + sum_all(root.rchild)