code: public synchronized K lastKey() {
  		if (!validState) {
  			throw new InvalidStateException();
  		}
  		final LeafNode<K, V> node = findSideLeafNode(false);
  		if (node == null) {
  			return null;
  		}
  		return node.keys[node.allocated - 1];
  	}
docs: Returns the last (highest) key currently in this tree.
  
  @return key