def __init__(self, values):
if len(values) == 0:
self.top = None
return
self.top = Node(values[-1])
if len(values) == 1:
self.bottom = self.top
prev = self.top
for val in values[1::-1]:
node = Node(val)
node.prev = prev
prev.next = node
prev = node
self.bottom = node
