众猿周知，万物皆可递归

public ListNode Merge(ListNode l1,ListNode l2) {
        if (l1 == null || l2 == null) return l2 == null ? l1 : l2;
        ListNode head = (l1.val<=l2.val) ? l1 : l2;
        head.next = Merge(head.next,head == l1 ? l2 : l1);
        return head;
    }

众猿亦知，可递归就可迭代

public ListNode Merge(ListNode l1, ListNode l2) {
        ListNode dum = new ListNode(0);
        ListNode cur = dum;
        while (l1!=null&&l2!=null){
            if (l1.val<=l2.val) {
                cur.next = l1;
                l1 = l1.next;
            }else{
                cur.next = l2;
                l2 = l2.next;
            }
            cur = cur.next;
        }
        if (l1==null) cur.next = l2;
        if (l2==null) cur.next = l1;
        return dum.next;
    }