WITH input AS
    (SELECT doc.extract('/l/text()').getNumberVal() n, count(*) over () cnt
     FROM
     TABLE(xmlSequence(extract(XMLType('<doc><l>'||
     replace(:input_data,',','</l><l>')||'</l></doc>'),'/doc/l'))) doc
    )
,t2(n1,n2,n3,lvl) AS (
    SELECT n,0,0,1 FROM input
    UNION ALL
    SELECT t2.n1
          ,DECODE(lvl,1,input.n,t2.n2)
          ,DECODE(lvl,2,input.n,t2.n3)
          ,lvl+1
      FROM t2,input
     WHERE lvl<3 AND input.n NOT IN (n1,n2)
    )
,b AS (SELECT n1,n2,n3 from t2 WHERE lvl=3 AND n2*2=n1+n3)
,t(n,str,cnt,lvl,root) AS
    (SELECT n,','||n||',',cnt,1,n
       FROM input
     UNION ALL
     SELECT input.n,t.str||input.n||',',input.cnt,lvl+1,t.root
       FROM t,input
      WHERE lvl<input.cnt
            AND INSTR(str,','||input.n||',')=0 --- nocycle
            AND (lvl<2 OR lvl>=2 AND t.n*2 != input.n + root)
            AND NOT EXISTS
                (SELECT 1
                  FROM b
                 WHERE instr(t.str||input.n||',', ','||b.n2||',') > instr(t.str||input.n||',', ','||b.n1||',')
                   AND instr(t.str||input.n||',', ','||b.n3||',') > instr(t.str||input.n||',', ','||n2||',')
                   AND instr(t.str||input.n||',', ','||b.n1||',') > 0
                )
    )
SELECT trim(BOTH ',' FROM str) AS No_Avg_Spanned_str
  FROM t
 WHERE lvl=cnt;

NO_AVG_SPANNED_STR
-----------------------------
4,2,3,1
1,3,4,2
1,3,2,4
3,1,4,2
3,1,2,4
3,4,1,2
2,1,4,3
2,4,1,3
2,4,3,1
4,2,1,3

10 rows selected.