Optimize GetLast() in DoublyLinkedList

[sammychien]

The GetLast() function traverses through the entire LL to get to the last pointer, when there's already a tail node. The fix gets rid of the traversal and returns the tail node value.

func (list *LinkedList) GetLast() (int, bool) {
	if list.head == nil {
		return 0, false
	}
	return list.tail.data, true
}