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(V)
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(V)
def add_edge ( self , U , V ) : 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 )
:param U:
    :param V:
    :return:
: param U : : param V : : return :