python3 -三个指针题解 | #反转链表#

# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None
#
# 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
#
# 
# @param head ListNode类 
# @return ListNode类
#
class Solution:
    def ReverseList(self , head: ListNode) -> ListNode:
        # write code here
        #1
        pre = None
        slow = head
        fast = head
        #2
        while fast:
            fast = fast.next
            slow.next = pre
            pre = slow
            slow = fast
        #3
        return pre