gistfile1.txt

    class MyLinkedList
    {
        class Link
        {
            Link prev;
            Link next;
            int value;

            public Link(int value)
            {
                this.value = value;
            }
        }

        /**
         * Initialize your data structure here.
         */
        Link head;
        Link tail;
        int size;

        public MyLinkedList()
        {
            head = new Link(Integer.MIN_VALUE);
            tail = new Link(Integer.MAX_VALUE);
            head.next = tail;
            tail.prev = head;
        }

        /**
         * Get the value of the index-th node in the linked list. If the index is invalid, return -1.
         */
        public int get(int index)
        {
            if (index < 0 || index >= size)
            {
                return -1;
            }

            Link current = head;
            while (index-- >= 0)
            {
                current = current.next;
            }
            return current.value;
        }

        /**
         * 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.
         */
        public void addAtHead(int val)
        {
            Link link = new Link(val);
            //-->
            head.next.prev = link;
            link.next = head.next;
            //<--
            head.next = link;
            link.prev = head;
            size++;
        }

        /**
         * Append a node of value val to the last element of the linked list.
         */
        public void addAtTail(int val)
        {
            Link link = new Link(val);
            //<--
            tail.prev.next = link;
            link.prev = tail.prev;
            //-->
            link.next = tail;
            tail.prev = link;
            size++;
        }

        /**
         * 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.
         */
        public void addAtIndex(int index, int val)
        {
            if (index < 0 || index > size)
            {
                return;
            }
            else if (index == 0)
            {
                addAtHead(val);
                return;
            }
            else if (index == size)
            {
                addAtTail(val);
                return;
            }

            Link current = head;
            Link link = new Link(val);
            while (index-- > 0)
            {
                current = current.next;
            }


            //-->
            link.next = current.next;
            current.next.prev = link;
            //<--
            current.next = link;
            link.prev = current;

            size++;
        }

        /**
         * Delete the index-th node in the linked list, if the index is valid.
         */
        public void deleteAtIndex(int index)
        {
            if (index < 0 || index >= size)
            {
                return;
            }

            Link current = head;
            while (index-- >= 0)
            {
                current = current.next;
            }

            current.prev.next = current.next;
            current.next.prev = current.prev;
            size--;
        }
    }