Question

I have a class called LinkStrand which functions very similarly to a Linked List. It has toString(), size(), append(), next(), and value(), but NOT a previous() method. I am trying to write code that reverses the order of the nodes, as well as the string within each node. In order to make it easier on myself in a few of the other methods I’ve had to write, I got rid of the next node requirement in constructing a node. Here’s what the Node class looks like:

private class Node {
    public Node myNext;
    public String myData;

    Node(String value) {
        myData = value;
        //myNext = next;
    }   
}

My .reverse() method currently reverses all of the strings within the nodes individually, but does not reverse the order of the nodes themselves. It is copied below:

public IDnaStrand reverse() {
    if (this == null)
        return this;
    Node prevStrand = null;
    Node thisStrand = myHead;
    String revString;
    LinkStrand val = new LinkStrand();
    while (thisStrand != null){
        Node hold = thisStrand.myNext;
        if (revSave.containsKey(thisStrand.myData)){
            revString = revSave.get(thisStrand.myData);
            val.append(revString);
            //System.out.println("Val is: " + val);
        }
        else{
            revString = reverseStr(thisStrand.myData);
            val.append(revString);
            //System.out.println("Val is: " + val);
            revSave.put(thisStrand.myData, revString);
        }
        thisStrand.myData = revString;
        thisStrand.myNext = prevStrand;
        prevStrand = thisStrand;
        thisStrand = hold;
    }
    return val;
}

I’ve been trying to come up with some kind of way to reverse the node order, but I’m drawing a blank. Does anyone have any idea how I might go about it?

Thanks!

Answer 1

If you are allowed to modify IDnaStrand and LinkStrand, add a method prepend(Node n). Then, as you iterate through the list, just prepend each node.

If you can’t modify your classes, save the Nodes to an array in reverse order (nodeArray[size-1], nodeArray[size-2], …) then create a new LinkStrand going through the array in order. Alternatively you could load the array in order, then create the LinkStrand in reverse order.

example:

thisStrand = myHead;
int size = 0;
while(thisStrand != null){
    thisStrand = thisStrand.myNext;
    size++;
}
Node[] nodeArray = new Node[size];
thisStrand = myHead;
for(int i = size-1, i < 0; i--) {
    nodeArray[i] = thisStrand;
}

Now you have the array, just load it into a new list! Of course adding a prepend method would be better, just have the class do

newElement.myNext = MyHead;
MyHead = newElement;