关于Java用链表实现减法的问题，如何解决？

Java用链表实现减法，咋判断是上第一个数比第二个数大还是第二个数比第一个数大，我判断过后，链表回不去了＞︿＜

    public Linkedlist sub(){
        Linkedlist res=new Linkedlist();Node head=new Node();Node pre=head;int ne=0;
        while (this.first.head!=null && this.second.head!=null) {
            if (compare()) {
                int ae=this.first.head.val-this.second.head.val-ne;
                if (ae<0) {ae=-ae;pre.next=new Node(ae);ne=1;}
                else{pre.next=new Node(ae);ne=0;}
                this.first.head=this.first.head.next;this.second.head=this.second.head.next;pre=pre.next;
            }
            else{
                int ae=-this.first.head.val+this.second.head.val-ne;if (ae<0) {ae=-ae;pre.next=new Node(ae);ne=1;}else{pre.next=new Node(ae);ne=0;}
                this.first.head=this.first.head.next;this.second.head=this.second.head.next;pre=pre.next;
            }
        }
        while (this.first.head==null && this.second.head!=null) {
            int ae=this.second.head.val-ne;if (ae<0) {ae=-ae;pre.next=new Node(ae);ne=1;}else{pre.next=new Node(ae);ne=0;}
            pre=pre.next;this.second.head=this.second.head.next;if (this.second.head==null) {pre.next=new Node('-');break;}
        }
        while (this.first.head!=null && this.second.head==null) {
            int ae=this.first.head.val-ne;if (ae<0) {ae=-ae;pre.next=new Node(ae);ne=1;}else{pre.next=new Node(ae);ne=0;}
            pre=pre.next;this.first.head=this.first.head.next;
        }
        if (ne==1) {pre.next=new Node('-');}
        return res;
    }
    public boolean compare(){
        Linkedlist te=this.first;Linkedlist de=this.second;
        while (te.head!=null && de.head!=null) {
            te.head=te.head.next;de.head=de.head.next;
        }
        if (te.head == null && de.head!=null) {return true;}
        else{return false;}
    //    if  else (this.first.head==null && this.second.head==null) {return false;}
        
    }

这是我的节点结构

class Node{
    Node next=null;
    int val;
    public Node(){}
    public Node(int val){
        this.val=val;
    }
    public Node(int val,Node next){
        this.val=val;this.next=next;
    }
}