public Key ceiling(Key key) {
	return ceiling(root, key);
}
	
private Key ceiling(Node node, Key key) {
	if (node == null)
		return null;
	int comp = key.compareTo(node.key);
	if (comp > 0)
		return ceiling(node.right, key);
	else if (comp == 0)
		return node.key;
	else {
		Key k = ceiling(node.left, key);
		return k == null? node.key: k;
	}
}