# Definition for singly-linked list.

# class ListNode:

#     def __init__(self, x):

#         self.val = x

#         self.next = None

class Solution:

    def reverseList(self, head: 'ListNode') -> 'ListNode':

        # cur当前节点

        # pre为当前节点的上一个节点，反转后的下一个节点

        # nex为当前节点的下一个节点，反转后的上一个节点

        cur = head

        pre = None

        while cur:

            # 节点原地反转

            nex = cur.next

            cur.next = pre

            pre = cur

            # 进入下一个要反转的节点

            cur = nex

        return pre