19. Remove Nth Node From End of List

删除链表的倒数第k个节点。

• 时间复杂度O(n)，空间复杂度O(1)
• Runtime: 80 ms, faster than 66.42%
• Memory Usage: 40.6 MB, less than 5.40%

/**
 * 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 dummy = new ListNode(0);
  dummy.next = head;
  let slow = dummy;
  let fast = dummy;
  while (fast && n >= 0) {
    fast = fast.next;
    n--;
  }
  while (fast) {
    fast = fast.next;
    slow = slow.next;
  }
  if (slow.next) slow.next = slow.next.next;
  return dummy.next;
};