19.删除链表的倒数第-n-个结点 ​

/**
 * 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 ret = new ListNode(-1, head);
  let fast = ret;
  let slow = ret;
  while (n--) {
    fast = fast.next;
  }
  if (!fast) return ret.next;

  while (fast.next) {
    slow = slow.next;
    fast = fast.next;
  }
  slow.next = slow.next.next;
  return ret.next;
};

/**
 * 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 ret = new ListNode(-1, head);
  let fast = ret;
  let slow = ret;
  while (n--) {
    fast = fast.next;
  }
  if (!fast) return ret.next;

  while (fast.next) {
    slow = slow.next;
    fast = fast.next;
  }
  slow.next = slow.next.next;
  return ret.next;
};