leetcode-19 删除链表的倒数第N个节点

/**
 * 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 length = 0;
  let cur = head;
  // 计算length
  while (cur) {
    length++;
    cur = cur.next;
  }
  let target = length - n - 1;
  cur = head;
  while (target > 0) {
    cur = cur.next;
    target--;
  }
  if (target === 0) {
  // 删除第二个节点
    if (cur.next && cur.next.next) {
        // 第二个节点不是最后一个节点
      cur.next = cur.next.next;
    } else {
        // 第二个节点是最后一个节点
      cur.next = null;
    }
  } else if (target === -1) {
    // 删除头节点
    head = head.next;
  } else if (cur.next) {
    cur.next = cur.next.next;
  }
  return head;
};