就地反转链表

头插法

LinkNode* ReverseLinkList(LinkNode* head)
{
  if(head == NULL) return NULL;
  LinkNode* pre = NULL;
  LinkNode* next = NULL;
  while(head)
  {
      next = head->next;
      head->next = pre;
      pre = head;
      head = next;
  }
  return pre;
}