力扣19.删除链表的倒数第 N 个结点

输入：head = [1,2,3,4,5], n = 2
输出：[1,2,3,5]

输入：head = [1], n = 1
输出：[]

输入：head = [1,2], n = 1
输出：[1]

 1 class Solution
 2 {
 3 public:
 4     ListNode *removeNthFromEnd(ListNode *head, int n)
 5     {
 6         ListNode *p=head;
 7         int length=0;
 8         while (p)
 9         {
10             p=p->next;
11             length++;
12         }
13         p=head;
14         for (int i=0;i<length-n-1;++i){
15             p=p->next;
16         }
17         if (n==length) //删除头节点
18             head=head->next;
19         else if (n==1) //删除尾节点
20             p->next=NULL;
21         else
22             p->next=p->next->next;
23         return head;
24     }
25 };