def add_head(self, value):
'''
Add value, before first node.
'''
new_node = List_Node(value)
new_node._next = self._head
self._head = new_node
if self._tail is None:
# If list used to be empty, update tail pointer
self._tail = self._head
def add_head(self, value):
'''
Add value into node just after head sentinel
'''
# EXERCISE 4: No special case for empty list
# Modifications:
# a) new_node._next should point to self._head._next, instead of self._head
# b) self._head._next should be changed, instead of self._head
# c) DELETE code that checks an empty tail.
new_node = List_Node(value)
new_node._next = self._head._next
self._head._next = new_node
return self._head
def add_tail(self, value):
'''
Add value into node after the tail
'''
# print('--------')
# print('Entering add_tail(', value ,')')
new_node = List_Node(value)
# print('before, list is:')
# print(repr(self))
# print('new node:', repr(new_node))
if self._tail is None:
# print('add_tail: empty list')
# Special case: empty list
# Create a list with a ONE node
self._tail = new_node
self._head = new_node
else:
# print('add_tail: general case')
# General case: add after tail, change tail pointer
self._tail._next = new_node
self._tail = new_node
def __init__(self, data=None):
'''
List constructor. Can be invoked in three ways:
Sentinel_List() makes an empty list.
Sentinel_List(<another Sentinel_List>) makes a copy of the other list.
Sentinel_List(<a python list>) makes list containing data's values.
'''
# EXERCISE 0 (done for you): create sentinel nodes
self._head = List_Node()
self._tail = List_Node()
# link sentinels together.
self._head._next = self._tail
if data != None:
if type(data) in (list, Sentinel_List):
for x in data:
self.add_tail(x)
else:
raise ValueError("Can't build list from data of type " +
str(type(data)))
def add_tail(self, value):
'''
Add value into node at the end of the list
'''
# EXERCISE 5: Insert before tail sentinel
# Modifications:
# a) set new_node._next = self._tail
# b) delete all code after that (it's all different)
# c) walk down the list:
# - prev starts at self._head
# - keep going while prev._next is not self._tail
# - after loop, prev._next should now be new_node
new_node = List_Node(value) # Keep this line
# Put a) here, and remove the lines below this one.
new_node._next = self._tail
_prev = self._head
while _prev._next is not self._tail:
_prev = _prev._next
_prev = new_node
return self._head
def insert(self, value, index):
'''
Insert a value at a given index
'''
# Check if index is valid
if index < 0 or index >= self.size():
raise IndexError(str(index))
if self._head == None:
# Special case: empty list
# Index will be zero (guaranteed), so just add to front
self.add_head(value)
else:
# Walk down the list, stopping just before
# insertion point
prev = self._head
for i in range(index - 1):
prev = prev._next
prev._next = List_Node(value, prev._next)
def insert(self, value, index):
'''
Insert a value at a given index
'''
# Check if index is valid
if index < 0 or index >= self.size():
raise IndexError(str(index))
# EXERCISE 8: skip head sentinel
# Modifications:
# a) Delete the special-case code (empty list)
# b) Delete the else: (of course)
# c) De-indent the code after the else (of course)
# d) Take one more step: (range(index) instead of range(index-1))
# Walk down the list, stopping just before
# insertion point
prev = self._head
for i in range(index):
prev = prev._next
prev._next = List_Node(value, prev._next)