反转链表

python 算法

思路：
1. 创建一个新的头节点 pre
2. head 节点指向 pre
3. 循环

改变指针方向

1->2->3->4
1<-2<-3-<4

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