Python数据结构之翻转链表

翻转一个链表

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