Python数据结构之翻转链表

样例：给出一个链表1->2->3->null，这个翻转后的链表为3->2->1->null

一种比较简单的方法是用“摘除法”。就是先新建一个空节点，然后遍历整个链表，依次令遍历到的节点指向新建链表的头节点。

那样例来说，步骤是这样的：

1. 新建空节点：None
2. 1->None
3. 2->1->None
4. 3->2->1->None

代码就非常简单了：

""" 
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

当然，还有一种稍微难度大一点的解法。我们可以对链表中节点依次摘链和链接的方法写出原地翻转的代码：

""" 
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 
   # 把摘链的地方连起来 
   pre.next = cur.next 
   cur = pre.next 
   temp.next = dummy.next 
   dummy.next = temp 
  return dummy.next 
  # write your code here

需要注意的是，做摘链的时候，不要忘了把摘除的地方再连起来

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