public ListNode Merge(ListNode list1, ListNode list2) {
    if (list1 == null)
        return list2;
    if (list2 == null)
        return list1;
    if (list1.val <= list2.val) {
        list1.next = Merge(list1.next, list2);
        return list1;
    } else {
        list2.next = Merge(list1, list2.next);
        return list2;
    }
}

public ListNode Merge(ListNode list1, ListNode list2) {
    ListNode head = new ListNode(-1);
    ListNode cur = head;
    while (list1 != null && list2 != null) {
        if (list1.val <= list2.val) {
            cur.next = list1;
            list1 = list1.next;
        } else {
            cur.next = list2;
            list2 = list2.next;
        }
        cur = cur.next;
    }
    if (list1 != null)
        cur.next = list1;
    if (list2 != null)
        cur.next = list2;
    return head.next;
}