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