剑指offer——反转链表

# -*- 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
        a=pHead
        b=pHead.next
        a.next=None
        while b :
            c=b.next
            b.next=a
            a=b
            b=c
        return a