# up.
next = tree.right or tree.parent
else: # Done with both children, so move up.
next = tree.parent
prev, tree = tree, next
return result
# @exclude
# 3
# 2 5
# 1 4 6
root = BinaryTreeNode(3)
root.parent = None
assert inorder_traversal(root) == [3]
root.left = BinaryTreeNode(2)
root.left.parent = root
root.left.left = BinaryTreeNode(1)
root.left.left.parent = root.left
assert inorder_traversal(root) == [1, 2, 3]
root.right = BinaryTreeNode(5)
root.right.parent = root
root.right.left = BinaryTreeNode(4)
root.right.left.parent = root.right
root.right.right = BinaryTreeNode(6)
root.right.right.parent = root.right
assert inorder_traversal(root) == [1, 2, 3, 4, 5, 6]
