class Solution:
  def reverseList(self, head):
    cur, prev = head, None
    while cur:
      temp = cur.next
      cur.next = prev
      prev = cur
      cur = temp
    return prev

class Solution:
  def reverseList(self, head):
    if head == None or head.next == None:
      return head
    prev = None
    cur = head
    post = head.next

    while post:
      cur.next = prev
      prev = cur
      cur = post
      post = post.next
    cur.next = prev
    return cur