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

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