Single rotation, either 0 (left) or 1 (right).

    Figure:
                a,0 (left)
                ---------->
          a                   b
           \                /   \
            b             a       c
             \
              c

    a = root
    save = root.right

def avl_rotate_single(root, direction):
    """
    Single rotation, either 0 (left) or 1 (right).

    Figure:
                a,0 (left)
                ---------->
          a                   b
           \                /   \
            b             a       c
             \
              c

    a = root
    save = root.right
    """
    other_side = 1 - direction
    save = root[other_side]
    save.parent = root.parent
    # root[other_side] = save[direction]
    # save[direction] = root
    root.set_child(other_side, save[direction])
    save.set_child(direction, root)
    rlh = height(root.left)
    rrh = height(root.right)
    slh = height(save[other_side])
    root.balance = max(rlh, rrh) + 1
    save.balance = max(slh, root.balance) + 1
    return save