select ename
  from emp
minus
select ename from emp where empno in (select mgr from emp);

select *
  from emp ee
 where not exists (select 1 from emp e where ee.empno = e.mgr) order by ename 

But still my interviewer  is asked for another alternate 

SELECT ename FROM emp 
WHERE empno IN (SELECT empno FROM emp MINUS SELECT mgr FROM emp)

SELECT ename FROM emp e
WHERE NOT EXISTS (SELECT NULL FROM emp m WHERE e.empno = m.mgr)

SELECT ename FROM emp WHERE empno NOT IN (SELECT NVL(mgr,0) FROM emp)

SELECT e.ename 
FrOM emp e LEFT OUTER JOIN emp m ON e.empno = m.mgr
WHERE m.mgr IS NULL

SELECT ename FROM emp
WHERE connect_by_isleaf = 1
CONNECT BY prior empno = mgr
START WITH mgr IS NULL