Flip list of Python data structures

Flip 1 list

Example: give a linked list 1- > 2- > 3- > null, this flipped list is 3 minus > 2- > 1- > null

A simpler method is to use the method of removal. It is to create a new empty node, then traverse the whole list, and make the traversed nodes point to the header node of the new list.

In that case, the steps are as follows:

1. New empty node: None
2. 1- > None
3. 2- > 1- > None
4. 3- > 2- > 1- > None

The code is very simple:

""" 
Definition of ListNode 
 
class ListNode(object): 
 
 def __init__(self, val, next=None): 
  self.val = val 
  self.next = next 
""" 
class Solution: 
 """ 
 @param head: The first node of the linked list. 
 @return: You should return the head of the reversed linked list. 
     Reverse it in-place. 
 """ 
 def reverse(self, head): 
  temp = None 
  while head: 
   cur = head.next 
   head.next = temp 
   temp = head 
   head = cur 
  return temp 
  # write your code here 

And, of course, there's another solution that's a little bit harder. We can write the code of in-situ inversion for the method of successively unchaining and linking nodes in the linked list:

""" 
Definition of ListNode 
 
class ListNode(object): 
 
 def __init__(self, val, next=None): 
  self.val = val 
  self.next = next 
""" 
class Solution: 
 """ 
 @param head: The first node of the linked list. 
 @return: You should return the head of the reversed linked list. 
     Reverse it in-place. 
 """ 
 def reverse(self, head): 
  if head is None: 
   return head 
  dummy = ListNode(-1) 
  dummy.next = head 
  pre, cur = head, head.next 
  while cur: 
   temp = cur 
   #  Connect the places where the chains are off  
   pre.next = cur.next 
   cur = pre.next 
   temp.next = dummy.next 
   dummy.next = temp 
  return dummy.next 
  # write your code here 

It's important to remember that when you do the chain pick, don't forget to reconnect where you took it off

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