707\. Design Linked List

Medium

Design your implementation of the linked list. You can choose to use a singly or doubly linked list.  
A node in a singly linked list should have two attributes: `val` and `next`. `val` is the value of the current node, and `next` is a pointer/reference to the next node.  
If you want to use the doubly linked list, you will need one more attribute `prev` to indicate the previous node in the linked list. Assume all nodes in the linked list are **0-indexed**.

Implement the `MyLinkedList` class:

*   `MyLinkedList()` Initializes the `MyLinkedList` object.
*   `int get(int index)` Get the value of the <code>index<sup>th</sup></code> node in the linked list. If the index is invalid, return `-1`.
*   `void addAtHead(int val)` Add a node of value `val` before the first element of the linked list. After the insertion, the new node will be the first node of the linked list.
*   `void addAtTail(int val)` Append a node of value `val` as the last element of the linked list.
*   `void addAtIndex(int index, int val)` Add a node of value `val` before the <code>index<sup>th</sup></code> node in the linked list. If `index` equals the length of the linked list, the node will be appended to the end of the linked list. If `index` is greater than the length, the node **will not be inserted**.
*   `void deleteAtIndex(int index)` Delete the <code>index<sup>th</sup></code> node in the linked list, if the index is valid.

**Example 1:**

**Input** 

["MyLinkedList", "addAtHead", "addAtTail", "addAtIndex", "get", "deleteAtIndex", "get"] 

[[], [1], [3], [1, 2], [1], [1], [1]]

**Output:** [null, null, null, null, 2, null, 3]

**Explanation:** 

    MyLinkedList myLinkedList = new MyLinkedList(); 
    myLinkedList.addAtHead(1); 
    myLinkedList.addAtTail(3); 
    myLinkedList.addAtIndex(1, 2); // linked list becomes 1->2->3 
    myLinkedList.get(1); // return 2 
    myLinkedList.deleteAtIndex(1); // now the linked list is 1->3 
    myLinkedList.get(1); // return 3

**Constraints:**

*   `0 <= index, val <= 1000`
*   Please do not use the built-in LinkedList library.
*   At most `2000` calls will be made to `get`, `addAtHead`, `addAtTail`, `addAtIndex` and `deleteAtIndex`.