// Textbook fragment 07.19
  /** Removes a node with zero or one child. */
  public E remove(Position<E> v)
    throws InvalidPositionException {
    BTPosition<E> vv = checkPosition(v);
    BTPosition<E> leftPos = vv.getLeft();
    BTPosition<E> rightPos = vv.getRight();
    if (leftPos != null && rightPos != null)
      throw new InvalidPositionException("Cannot remove node with two children")
;
    BTPosition<E> ww;   // the only child of v, if any
    if (leftPos != null)
      ww = leftPos;
    else if (rightPos != null)
      ww = rightPos;
    else                // v is a leaf
      ww = null;
    if (vv == root) {   // v is the root
      if (ww != null)
        ww.setParent(null);
      root = ww;
    }
    else {              // v is not the root
      BTPosition<E> uu = vv.getParent();
      if (vv == uu.getLeft())
        uu.setLeft(ww);
      else
        uu.setRight(ww);
      if(ww != null)
        ww.setParent(uu);
    }
    size--;
    return v.element();
  }