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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