[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) {
  const dummyhead = new ListNode(-1);
  dummyhead.next = head;

  let fastindex = dummyhead;
  let slowindex = dummyhead;

  for (let i = 0; i < n + 1; i++) {
    fastindex = fastindex.next;
  }

  while (fastindex !== null) {
    fastindex = fastindex.next;
    slowindex = slowindex.next;
  }

  slowindex.next = slowindex.next.next;

  return dummyhead.next;
};