19.删除链表的倒数第N个结点

给你一个链表，删除链表的倒数第 n 个结点，并且返回链表的头结点。

示例 1：

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

输入：head = [1], n = 1
输出：[]
示例 3：

输入：head = [1,2], n = 1
输出：[1]

提示：

链表中结点的数目为 sz
1 <= sz <= 30
0 <= Node.val <= 100
1 <= n <= sz

 1 /**
 2  * Defination for singly-linked list
 3  * function ListNode(val,next){
 4  * this.val=(val===undefined?0:val)
 5  * this.next=(next===undefined?null:next)
 6  * }
 7  */
 8 /**
 9  * @param {ListNode} head
10  * @param {number} n
11  * @return {ListNode}
12  */
13 
14 var removeNthFromEnd = function(head, n) {
15     let dummy = new ListNode();
16     dummy.next = head;
17     let n1 = dummy;
18     let n2 = dummy;
19     for (let i = 0; i <= n; i++) {
20         n2 = n2.next;
21     }
22     while (n2 !== null) {
23         n1 = n1.next;
24         n2 = n2.next;
25     }
26     n1.next = n1.next.next;
27     return dummy.next;
28 };