一日一练：设计链表

设计链表的实现。您可以选择使用单链表或双链表。单链表中的节点应该具有两个属性：val 和 next。val 是当前节点的值，next 是指向下一个节点的指针/引用。如果要使用双向链表，则还需要一个属性 prev 以指示链表中的上一个节点。假设链表中的所有节点都是 0-index 的。

var MyLinkedList = function () {
  this.len = 0
  this.dummy = new ListNode(-1, null)
}

function ListNode(val, next) {
  this.val = val
  this.next = next
}

/**
 * @param {number} index
 * @return {number}
 */
MyLinkedList.prototype.get = function (index) {
  if (index < this.len && index >= 0) {
    let cur = this.dummy.next
    while (index-- > 0) {
      cur = cur.next
    }
    return cur.val
  }
  return -1
}

/**
 * @param {number} val
 * @return {void}
 */
MyLinkedList.prototype.addAtHead = function (val) {
  this.addAtIndex(0, val)
}

/**
 * @param {number} val
 * @return {void}
 */
MyLinkedList.prototype.addAtTail = function (val) {
  this.addAtIndex(this.len, val)
}

/**
 * @param {number} index
 * @param {number} val
 * @return {void}
 */
MyLinkedList.prototype.addAtIndex = function (index, val) {
  if (index <= this.len) {
    let pre = this.dummy
    let count = Math.max(0, index)
    while (count-- > 0) {
      pre = pre.next
    }

    if (index === this.len) {
      pre.next = new ListNode(val, null)
    } else {
      pre.next = new ListNode(val, pre.next)
    }
    this.len++
  }
}

/**
 * @param {number} index
 * @return {void}
 */
MyLinkedList.prototype.deleteAtIndex = function (index) {
  if (index < this.len && index >= 0) {
    let pre = this.dummy
    while (index-- > 0) {
      pre = pre.next
    }
    if (pre.next) {
      pre.next = pre.next.next
    } else {
      pre.next = null
    }
    this.len--
  }
}