find the max node when node regard as a root node   
        :param node: 
        :return: max node

def maximum(self, node):
        """
        find the max node when node regard as a root node   
        :param node: 
        :return: max node 
        """
        temp_node = node
        while temp_node.right is not None:
            temp_node = temp_node.right
        return temp_node