SQL> 
SQL> select replace(regexp_substr(n.vv,'.+?,',1,level),',','') id,replace(regexp_substr(n.vv,'.+?,',1,level+1),',','') parent_id
  2    from (select '4,3,2,1,' vv from dual
  3          /*union all
  4          select '6,5,1,' vv from dual
  5          union all
  6          select '8,5,' vv from dual
  7          union all
  8          select '1,' vv from dual
  9          */) n
 10   connect by level <length(trim(n.vv)) - length(trim(replace(n.vv, ',')))+1
 11  ;
ID       PARENT_ID
-------- ---------
4        3
3        2
2        1
1        

SQL> 

ID       PARENT_ID
-------- ---------
4        3
3        2
2        1
1        
6        5
5        
8        5
5        
1        
9 rows selected