class Solution:
    def reverseList(self, head):
        #iteration
        if not head:
            return head
        pre = None
        while head:
            nxt = head.next
            head.next = pre # 转向
            # move
            pre = head
            head = nxt
        return pre

        #recursion 
        if not head or not head.next:
            return head
        new_head = self.reverseList(head.next)

        head.next.next = head # 转向
        head.next = None
        return new_head