public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode dummy=new ListNode(-1);
        ListNode head=dummy;
        if(l1==null){
            return l2;
        }
        if(l2==null){
            return l1;
        }
        while(l1!=null && l2!=null){
            if(l1.val<l2.val){
                head.next=l1;
                l1=l1.next;
                head=head.next;
            }else{
                head.next=l2;
                l2=l2.next;
                head=head.next;
            }
        }
        
        if(l1==null){
            head.next=l2;
        }
        if(l2==null){
            head.next=l1;
        }
        
        return dummy.next;
    }