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 public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode res = new ListNode();
        ListNode tail = res;
			while (l1 != null || l2 != null) {
				if (l2 == null || (l1 != null && l1.val < l2.val)) {
					tail.next = l1;
					tail = l1;
					l1 = l1.next;
				}else {
					tail.next = l2;
					tail = l2;
					l2 = l2.next;
				}
			}
			return res.next;
    }