def _append(self,x):
"""appends x onto end of the list
post: x is appended onto the end of the list"""
newNode=DListNode(x)
#General Case (not empty)
if self.head is not None:
node=self.tail
newNode.link = node.link
node.link=newNode
newNode.rev=node
#Empty MEthod
else:
self.head=newNode
self.tail=newNode
self.size+=1
def insert(self,index,value):
'''
pre: 0<=index<self.size, value should be an integer or a float or a string
post: insert a new node whose value is value at the position index in the list
Exception: IndexError if pre violated
'''
#IndexError if Pre Violated
if not (0 <= abs(index) <= self.size):
raise IndexError
#Convert the index to the positive version
if index < 0:
index = self.size+index
#If the index is the last position
# apply append directly
if index == self.size:
self._append(value)
return
#Other Cases
newNode = DListNode(value)
#when index is the 0, first position
if index == 0:
node = self._find(0)
newNode.link = self.head
self.head = newNode
node.rev = newNode
#general case
else:
node = self._find(index)
prev = node.rev
newNode.link = prev.link
prev.link = newNode
newNode.rev = prev
node.rev = newNode
self.size += 1
