text:  class Solution:
      def XXX(self, head: ListNode, n: int) -> ListNode:
          pn = p = head
          for i in range(n):
              p = p.next
          if p == None:
              # 说明删除的是第一个元素
              return head.next
          while True:
              if p == None or p.next == None:
                  break
              else:
                  p = p.next
                  pn = pn.next
          if pn.next == None:
              head = None
          else:
              pn.next = pn.next.next
          return head
  
  