# 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:
        pre, res = head, None
        while pre:
            res, res.next, pre = pre, res, pre.next
        return res
