面试题24：反转链表

题目描述：

输入一个链表，反转链表后，输出新链表的表头。

解题思路：

为防止链表出现断裂的情况，需要定义3个指针，一个指向当前遍历的节点current，以及它的前一个节点previous和后一个节点later

# -*- coding:utf-8 -*-
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None
class Solution:
    # 返回ListNode
    def ReverseList(self, pHead):
        # write code here
        if pHead==None:
            return None
        current = pHead
        previous = None
        later = None
        while current!=None:
            later = current.next
            current.next = previous
            previous = current
            current = later
        return previous