Question

I am trying to implement LinkedList using Java, just to test my skills. I am stuck at one problem, where I have to append two linked lists that I have created. I get into an infinite loop here. Is there any way I can improve the code and implement the desired output ?

I/Ps :

List A : 4->3->2->1->0

List B : 4->3->2->1->0

O/P should be : 4->3->2->1->0->4->3->2->1->0

class List {
    int val;
    List next;

    public List(int val) {
        this.val = val;
    }

    public String toString() {
        String output = "";
        List current = this;

        while (current != null) {
            output += current.val + "->";
            current = current.next;
        }
        return output + "NULL";
    }
}


class AppendLinkedLists {

    static List push(List list, int num) {
        List newList = new List(num);
        newList.next = list;
        return newList;
    }

    static List appendLists(List listA, List listB) {
        if (listA == null)
            return listB;
        else {
            List tempList = listA;
            while (tempList.next.next != null) {
                tempList = tempList.next;
            }
            tempList.next.next = listB;
            return listA;           
        }
    }

    public static void main(String[] args) {
        List listA = new List(0);
        listA = push(listA, 1);
        listA = push(listA, 2);
        listA = push(listA, 3);
        listA = push(listA, 4);

        List listB = listA;

        System.out.println("Input List A : " + listA.toString());
        System.out.println("Input List B : " + listB.toString());
        listA = appendLists(listA, listB);
        System.out.println("Combined Input Lists A and B : " + listA.toString());
    }
}

Answer 1

You don’t have 2 lists. You only have one.

   List listB = listA;

assigns the reference listB to point to listA. So you’re appending listA onto itself.

Regardless of what other issues you have, I would correct this (the simplest way being to create a listB in a similar fashion to how you’ve created listA).

My other comment (hope you don’t mind) is that you’ve created a List object, but it has little/no behaviour of its own. Instead of creating your AppendLinkedLists class, I would put the functionality into the List object e.g. instead of:

listA = push(listA, 1);

write:

listA.push(1);

etc. So the behaviour is encapsulated within the List object. Similarly you could then write:

listA.push(listB);

by making use of overloading.