Question

I’m trying to create a Stack to take a string and add each of the strings characters to it, but I was told it would be far more efficient use a LinkedList. How would I use a LinkedList to create and manipulate a stack?

An example would be very appreciated!

Answer 1

Ok, the problem is that you’re not using First at all. Try the following:

public class Example
{
    private LinkedList aList = new LinkedList();

    public void push(char c) {
        aList.addFirst(c);
    }
    public Object pop() {
        return aList.removeFirst();
    }
    public boolean empty() {
        return aList.isEmpty();
    }
    public static void main(String[] args)  {
        Stack exmpStack = new Stack();
        String ranString = "Dad";
        for (int i = 0; i < ranString.length(); i++)  {
            exmpStack.push(ranString.charAt(i));
        }
        while (!exmpStack.empty())  {
            System.out.print(exmpStack.pop());
        }
    }
}

Because you never use First it’s always null – so your loop never runs at all! Instead of using that at all, just use the build in isEmpty() function.

Edit: Of course, you don’t really need those functions at all – the following will work fine:

public class Example
{
    private LinkedList aList = new LinkedList();

    public static void main(String[] args)  {
        String ranString = "Dad";
        for (int i = 0; i < ranString.length(); i++)  {
            aList.push(ranString.charAt(i));
        }
        while (!aList.isEmpty())  {
            System.out.print(aList.pop());
        }
    }
}

Now this is still a bit unsafe – you can go one step further by using the following:

private LinkedList<Character> aList = new LinkedList<>();

That way it’s a bit safer, and returns Characters instead of Objects – and Characters can be implicitly cast to char 🙂