19. 删除链表的倒数第 N 个结点

快慢指针

/**
 * 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 result = new ListNode()
  result.next = head
  let first = head,
    second = head
  let i = 0
  while (first) {
    first = first.next
    if (i > n) {
      second = second.next
    }
    i++
  }
  if (n > i) {
    return null
  }
  if (i === n) {
    result.next = head.next
  } else {
    second.next = second.next.next
  }
  return result.next
}