Question

So I was given this code, which to my knowledge I am not allowed to change:

public void insertionSort () {
    if (head == tail) {
        // empty list is sorted
        return;
    }
    Node nextNode = head.next; // start inserting second node
    while (nextNode != tail) {
        // set insertNode to node to be inserted in this iteration
        Node insertNode = nextNode; 
        // set nextNode to node to be inserted in next iteration
        nextNode = nextNode.next;  

        // find position where insertNode has to be inserted
        Node current = insertNode.prev;
        while (current != null && insertNode.value.compareTo(current.value) < 0) {
            current = current.prev;
        }

        // insert insertNode after current
        insertNode.moveAfter(current);
    }
}

I am not very familiar with linked lists but from what I can tell if the second while loop operates on the first iteration then this code will pass null into moveAfter() So far for moveAfter() I have:

 /**
     * Inserts this node after the specified node. If this and the specified node 
     * refer to the same node in the list, the list remains unchanged. If the 
     * specified node current is null, then this node is inserted at the head of 
     * the list.
     *
     * Precondition: this Node and the specified node are two nodes of this list
     *               this node and the specified node are not equal to the tail
     * 
     * @param node - the node in this list after which this node is inserted
     */
    public void moveAfter (Node node) {
        if(this.prev == null && node.next == null){
            throw new NoSuchElementException();
        }
        if(node.prev == null && node.next==null){
            throw new NoSuchElementException();
        }
        if(this == tail){
            throw new NoSuchElementException();
        }
        if(node == tail){
            throw new NoSuchElementException();
        }

          this.prev.next = this.next;
          this.next = node.next;
          node.next = this;                            
          this.prev = node;
    }
}  

If I am correct that insertionSort() passes null to moveAfter() how can I rectify this and reset “current” to it original value if i can not change insertionSort();

*Side note: Apologies if my question is hard to read or not asked correctly. I seem to have a knack at screwing them up on this website.

Answer 1

The comments before your moveAfter() method state:

If the specified node current is null, then this node is inserted at the head of the list.

Since insertionSort() uses a head variable, I assume this is a member variable which you can also use in moveAfter() to do as the specification states.