code: def add_edge(self, U, V):
      '''
      :param U:
      :param V:
      :return:
      '''
      if not U in self.edges:
        self.edges[U] = set()
      if not V in self.edges:
        self.edges[V] = set()
      if not V in self.edges[U]:
        self.edges[U].add(Vf add_edge(self, U, V):
      '''
      :param U:
      :param V:
      :return:
      '''
      if not U in self.edges:
        self.edges[U] = set()
      if not V in self.edges:
        self.edges[V] = set()
      if not V in self.edges[U]:
        self.edges[U].add(V)
docs: :param U:
      :param V:
      :return: