反转链表

题目：定义一个函数，输入一个链表的头结点，反转该链表并输出反转后链表的头结点，链表节点定义如下：

class ListNode:
    def __init__(self, x):
        self.val = x
        self.next = None

思路一：新建一个链表，利用头插法反转该链表

class ListNode:
    def __init__(self, x):
        self.val = x
        self.next = None
class Solution:
    # 返回ListNode
    def ReverseList(self, pHead):
        p1=ListNode(0)
        p=pHead
        while p!=None:
            q=p.next
            p.next=p1.next
            p1.next=p
            p=q
        return p1.next

思路二：三个指针在链表上同时滑动。

# -*- coding:utf-8 -*-
class ListNode:
    def __init__(self, x):
        self.val = x
        self.next = None
class Solution:
    # 返回ListNode
    def ReverseList(self, pHead):
        head=pHead
        pre=None
        reversehead=None
        while head!=None:
            q=head.next
            if head==None:
                reversehead=head
            head.next=pre
            pre=head
            head=q
        return reversehead