将节点添加到最初为空的LinkedList

Question

I'm having issues when I try to add a node to a linked list that is initialized to null. In my method I set a test case to check if the node is initially null and if so it creates a new node with the value that was passed in. But for whatever reason it doesn't work unless the node has atleast one element already passed in. Check it out:

    Node addNode(Node node, int val)
    {
        if(node == null)
            {
                Node newNode = new Node(val);
                //node = newNode;
                return newNode;
            }
        node.next = addNode(node.next, val);
        return node;
    }

//Driver Class
    Scanner in = new Scanner(System.in);
    Node myNode = new Node(1);
    int numEntries = in.nextInt();
    for(int i = 0 ; i < numEntries ; i++)
    {
        int inputVal = in.nextInt();
        myNode.addNode(myNode, inputVal);
    }

The above code will not run if myNode is initialized to a null value (Node myNode = null;)

Full Code:

/* package whatever; // don't place package name! */

   import java.util.*;
   import java.lang.*;
   import java.io.*;




/* Name of the class has to be "Main" only if the class is public. */
    class Ideone
    {
     public static class Node
     {
         private int value;
         Node next;
         public Node()
         {
             next = null;
         }
         public Node(int val)
         {
            value = val;
            next = null;
        }
        Node addNode(Node node, int val)
        {
            if(node == null)
                {
                    Node newNode = new Node(val);
                    //node = newNode;
                    return newNode;
                }
            node.next = addNode(node.next, val);
            return node;
        }
    }

    public static void main (String[] args) throws java.lang.Exception
    {
        Scanner in = new Scanner(System.in);
        Node myNode = new Node(1);
        Node current = null;
        Node oddFirst = new Node(1);
        int numEntries = in.nextInt();
        for(int i = 0 ; i < numEntries ; i++)
        {
            int inputVal = in.nextInt();
            myNode.addNode(myNode, inputVal);
        }
        current = myNode;
        while(current != null) // Check if values were copied correctly
        {
            if(oddFirst == null) 
            {
                oddFirst = new Node(current.value);
            }
            oddFirst.addNode(oddFirst,current.value);
            //oddFirst = current.next;
            //oddFirst = oddFirst.next;
            current = current.next.next;
        }
        while(oddFirst != null)
        {
            System.out.println("Current Value: " + oddFirst.value);
            oddFirst = oddFirst.next;
        }
    }
}

Answer 1

A simple solution for a linked list:

class Node {
    int val;
    Node next;
}

public class LinkedList {
    public Node first;
    public Node last;
    public void addNext(int val) {
        Node node = new Node();
        node.val = val;
        if(last == null) {
            first = last = node;
        }
        else {
            last.next = node;
            last = node;
        }
    }
}

The main issue with the original code is that it doesn't concern itself with the case of the empty list. You cannot discern the case where the list is comprised of a single 1 value, and the empty list.

Answer 2

Because you're not handling the return value of addNode().

You're returning a Node in the following function:

Node addNode(Node node, int val)

but you're not handling the return here:

myNode.addNode(myNode, inputVal);

This should help you figure out the solution.