Post on 19-Oct-2020
Linked lists memory view
lst = Linked_list("abc")
name space
memory space
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type: Node type: Node type: Node type:
linked_list
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lst.reverse()
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Exercise: implement list reverse with O(1) additional memory
Guidance
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1. Think about “the middle” of the process.
You have 2 consecutive elements prev, curr.
All elements up to prev were reversed.
Write the loop’s body.
2. Think about the initialization of prev, curr
3. Think about termination of the loop and
extra actions after it
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Solution
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def reverse(self):
prev = None
curr = self.next
while curr !=None:
tmp = curr.next
curr.next = prev
prev = curr
curr = tmp
self.next = prev
>>> lst = Linked_list("abcde")
>>> lst
[a,36067440] [b,36067376] [c,36067024] [d,36067312] [e,36066864]
>>> lst.reverse()
>>> lst
[e,36066864] [d,36067312] [c,36067024] [b,36067376] [a,36067440]
Demo
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prev curr
Initialization
def reverse(self):
prev = None
curr = self.next
while curr !=None:
tmp = curr.next
curr.next = prev
prev = curr
curr = tmp
self.next = prev
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Demo
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prev curr
Step 1
def reverse(self):
prev = None
curr = self.next
while curr !=None:
tmp = curr.next
curr.next = prev
prev = curr
curr = tmp
self.next = prev
None
tmp
Demo
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prev curr
Step 1
def reverse(self):
prev = None
curr = self.next
while curr !=None:
tmp = curr.next
curr.next = prev
prev = curr
curr = tmp
self.next = prev
None
tmp
Demo
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prev curr
Step 1
def reverse(self):
prev = None
curr = self.next
while curr !=None:
tmp = curr.next
curr.next = prev
prev = curr
curr = tmp
self.next = prev
None
tmp
Demo
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prev curr
Step 1
def reverse(self):
prev = None
curr = self.next
while curr !=None:
tmp = curr.next
curr.next = prev
prev = curr
curr = tmp
self.next = prev
None
tmp
Demo
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prev curr
Step 2
def reverse(self):
prev = None
curr = self.next
while curr !=None:
tmp = curr.next
curr.next = prev
prev = curr
curr = tmp
self.next = prev
None
tmp
Demo
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prev curr
Step 3
def reverse(self):
prev = None
curr = self.next
while curr !=None:
tmp = curr.next
curr.next = prev
prev = curr
curr = tmp
self.next = prev
None
tmp
Demo
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Termination
def reverse(self):
prev = None
curr = self.next
while curr !=None:
tmp = curr.next
curr.next = prev
prev = curr
curr = tmp
self.next = prev
None
tmp