题目链接：https://leetcode-cn.com/problems/reverse-linked-list/
描述：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：
# Definition for singly-linked list.
# class ListNode(object):
# def __init__(self, x):
# self.val = x
# self.next = None
class Solution(object):
def reverseList(self, head):
"""
:type head: ListNode
:rtype: ListNode
"""
if head == None or head.next == None:
return head
cur = head
p = head.next
pre = None
while p:
cur.next = pre
pre = cur
cur = p
p = p.next
cur.next = pre
return cur
