text: # Definition for singly-linked list.
  # class ListNode:
  #     def __init__(self, val=0, next=None):
  #         self.val = val
  #         self.next = next
  class Solution:
      def reverseList(self, head: Optional[ListNode]) -> Optional[ListNode]:
          prev = None
          while head:
              curr = head
              head = head.next
              curr.next = prev
              prev = curr
          return prev