反转链表

反转一个单链表。

示例:

输入: 1->2->3->4->5->NULL
输出: 5->4->3->2->1->NULL

代码：

v.1

知道结构以后就很简单了，这是最简单的迭代。然而递归方式我还没有看懂。

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    def reverseList(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        pre = None
        while head is not None:
            temp = head.next
            head.next = pre
            pre = head
            head = temp

        return pre

v2.0

class Solution:
    def reverseList(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        if not head or not head.next:
            return head
        p = self.reverseList(head.next)
        head.next.next = head
        head.next = None
        return p