/*** Definition for singly-linked list.* function ListNode(val, next) {*     this.val = (val===undefined ? 0 : val)*     this.next = (next===undefined ? null : next)* }*/
/*** @param {ListNode} head* @param {number} n* @return {ListNode}*/
var removeNthFromEnd = function(head, n) {let empty_node=new ListNode(-1);empty_node.next=head;let p_slow=empty_node;let p_fast=empty_node;for(let i=0;i<=n;i++){p_fast=p_fast.next;}while(p_fast){p_fast=p_fast.next;p_slow=p_slow.next;}p_slow.next=p_slow.next.next;return empty_node.next;};