ListNode.java

public class ListNode {

    public int val;
    public ListNode next = null;

    public ListNode(int x) {
        val = x;
    }

    public ListNode (int[] arr){

        if(arr == null || arr.length == 0)
            throw new IllegalArgumentException("arr can not be empty");

        this.val = arr[0];
        ListNode curNode = this;
        for(int i = 1 ; i < arr.length ; i ++){
            curNode.next = new ListNode(arr[i]);
            curNode = curNode.next;
        }
    }

    ListNode findNode(int x){

        ListNode curNode = this;
        while(curNode != null){
            if(curNode.val == x)
                return curNode;
            curNode = curNode.next;
        }
        return null;
    }

    @Override
    public String toString(){

        StringBuilder s = new StringBuilder("");
        ListNode curNode = this;
        while(curNode != null){
            s.append(Integer.toString(curNode.val));
            s.append(" -> ");
            curNode = curNode.next;
        }
        s.append("NULL");
        return s.toString();
    }
}