code: def transplant(self, node_u, node_v):
          
          if node_u.parent is None:
              self.root = node_v
          elif node_u is node_u.parent.left:
              node_u.parent.left = node_v
          elif node_u is node_u.parent.right:
              node_u.parent.right = node_v
          # check is node_v is None 
          if node_v:
              node_v.parent = node_u.parent
docs: replace u with v
          :param node_u: replaced node
          :param node_v: 
          :return: None