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type node struct {
    val  int
    next *node
    prev *node
}

type MyLinkedList struct {
    head   *node
    tail   *node
    length int
}


/** Initialize your data structure here. */
func Constructor() MyLinkedList {
    return MyLinkedList{}
}


/** Get the value of the index-th node in the linked list. If the index is invalid, return -1. */
func (this *MyLinkedList) Get(index int) int {
    if index < 0 || index >= this.length  {
        return -1
    }
    for cur := this.head; ; cur = cur.next {   
        if index == 0 {
            return cur.val
        }
        index --
    }
}


/** 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. */
func (this *MyLinkedList) AddAtHead(val int)  {
    temp := &node{
        val : val,
        next: this.head,
        prev: nil,
    }
    if this.head == nil {
        this.tail = temp
    }else {
        this.head.prev = temp      
    }
     
    this.head = temp
    this.length ++
}


/** Append a node of value val to the last element of the linked list. */
func (this *MyLinkedList) AddAtTail(val int)  {
    temp := &node{
        val : val,
        next: nil,
        prev: this.tail,
    }
    if this.tail == nil {
        this.head = temp
    }else {
        this.tail.next = temp      
    }
     
    this.tail = temp
    this.length ++
}


/** Add a node of value val before the index-th node in the linked list. If index equals to the length of linked list, the node will be appended to the end of linked list. If index is greater than the length, the node will not be inserted. */
func (this *MyLinkedList) AddAtIndex(index int, val int)  {
    if index < 0 || index > this.length  {
        return
    }
    if index == 0 {
        this.AddAtHead(val)
        return
    }
    if index == this.length {
        this.AddAtTail(val)
        return
    }
    temp := &node{
        val : val,
        next: nil,
        prev: nil,
    }
    for cur := this.head; ; cur = cur.next {   
        if index == 0 {
            cur.prev.next = temp
            temp.next = cur
            temp.prev = cur.prev
            cur.prev =  temp           
            break
        }
        index --
    }
    this.length ++
}


/** Delete the index-th node in the linked list, if the index is valid. */
func (this *MyLinkedList) DeleteAtIndex(index int)  {
    if index < 0 || index >= this.length  {
        return
    } 
    if index == 0 {
        this.head = this.head.next
        if this.head != nil {
            this.head.prev = nil
        }else {
            this.tail = nil
        }
        this.length --
        return
    }
    if index == this.length - 1 {
        this.tail = this.tail.prev
        if this.tail != nil {
            this.tail.next = nil
        }else {
            this.head = nil
        }
        this.length --
        return
    }
    for cur := this.head; ; cur = cur.next {   
        if index == 0 {
            cur.prev.next = cur.next
            cur.next.prev = cur.prev          
            break
        }
        index --
    }
    this.length --
}


/**
 * Your MyLinkedList object will be instantiated and called as such:
 * obj := Constructor();
 * param_1 := obj.Get(index);
 * obj.AddAtHead(val);
 * obj.AddAtTail(val);
 * obj.AddAtIndex(index,val);
 * obj.DeleteAtIndex(index);
 */