反转一个单链表。
class Solution: def reverseList(self, head: ListNode) -> ListNode: if head is None or head.next is None: return head # pre 就是那个空链表 pre, cur = None, head # 不断将当前链表移动到空链表上 while cur:#cur是当前的点 next_node = cur.next#保存当前节点的后面内容 cur.next = pre#链接到等于新链表 pre = cur#赋给新链表 cur = next_node#再把cur往后移动 return pre