设计链表（JavaScript）

class LinkNode {
    constructor(val, next) {
        this.val = val;
        this.next = next;
    }
}

var MyLinkedList = function() {
    this._size = 0;
    this._tail = null;
    this._head = null;
};

/** 
 * @param {number} index
 * @return {number}
 */
MyLinkedList.prototype.getNode = function(index) {
    if (index < 0 || index >= this._size) return null;
    let cur = new LinkNode(0, this._head);
    while (index-- >= 0) {
        cur = cur.next;
    }
    return cur;
};
MyLinkedList.prototype.get = function(index) {
    if (index < 0 || index >= this._size) return -1;
    return this.getNode(index).val;
};

/** 
 * @param {number} val
 * @return {void}
 */
MyLinkedList.prototype.addAtHead = function(val) {
    const node = new LinkNode(val, this._head);
    this._head = node;
    this._size++;
    if (!this._tail) {
        this._tail = node;
    }
};

/** 
 * @param {number} val
 * @return {void}
 */
MyLinkedList.prototype.addAtTail = function(val) {
    const node = new LinkNode(val, null);
    this._size++;
    if(this._tail) {
        this._tail.next = node;
        this._tail = node;
        return;
    }
    this._tail = node;
    this._head = node;
};

/** 
 * @param {number} index 
 * @param {number} val
 * @return {void}
 */
MyLinkedList.prototype.addAtIndex = function(index, val) {
    if (index > this._size) return;
    if (index <= 0) {
        this.addAtHead(val);
        return;
    }
    if (index === this._size) {
        this.addAtTail(val);
        return;
    }
    const node = this.getNode(index - 1);
    node.next = new LinkNode(val, node.next);
    this._size++;
};

/** 
 * @param {number} index
 * @return {void}
 */
MyLinkedList.prototype.deleteAtIndex = function(index) {
    if(index < 0 || index >= this._size) return;
    if(index === 0) {
        this._head = this._head.next;
        // 如果删除的这个节点同时是尾节点，要处理尾节点
        if(index === this._size - 1){
            this._tail = this._head
        }
        this._size--;
        return;
    }
    // 获取目标节点的上一个的节点
    const node = this.getNode(index - 1);    
    node.next = node.next.next;
    // 处理尾节点
    if(index === this._size - 1) {
        this._tail = node;
    }
    this._size--;
};

/**
 * Your MyLinkedList object will be instantiated and called as such:
 * var obj = new MyLinkedList()
 * var param_1 = obj.get(index)
 * obj.addAtHead(val)
 * obj.addAtTail(val)
 * obj.addAtIndex(index,val)
 * obj.deleteAtIndex(index)
 */