CREATE TABLE ht AS
  SELECT 'A' c1, 'B' c2 FROM DUAL
  UNION
  SELECT 'C' c1, 'A' c2 FROM DUAL
  UNION
  SELECT 'B' c1, 'D' c2 FROM DUAL
  UNION
  SELECT 'X' c1, 'Y' c2 FROM DUAL
  UNION
  SELECT 'Y' c1, 'Z' c2 FROM DUAL;

WITH a AS
       (SELECT       CONNECT_BY_ROOT c1 c1, c2, CONNECT_BY_ISLEAF leaf, LEVEL lev
        FROM         ht a
        WHERE        CONNECT_BY_ISLEAF = 1
        CONNECT BY   PRIOR c2 = c1),
     b AS (SELECT a.*, MAX(lev) OVER (PARTITION BY c2) mlev FROM a)
SELECT   c1, c2
FROM     b
WHERE    lev = mlev;

SQL> WITH a AS
  2         (SELECT       CONNECT_BY_ROOT c1 c1, c2, CONNECT_BY_ISLEAF leaf, LEVEL lev
  3          FROM         ht a
  4          WHERE        CONNECT_BY_ISLEAF = 1
  5          CONNECT BY   PRIOR c2 = c1),
  6       b AS (SELECT a.*, MAX(lev) OVER (PARTITION BY c2) mlev FROM a)
  7  SELECT   c1, c2
  8  FROM     b
  9  WHERE    lev = mlev;
C C
- -
C D
X Z

SQL> SELECT       CONNECT_BY_ROOT c1 c1, c2
  2          FROM         ht a
  3          WHERE        CONNECT_BY_ISLEAF = 1
  4          CONNECT BY   PRIOR c2 = c1
  5  start with c1 not in (select c2 from ht)
  6  /
C C
- -
C D
X Z

DROP TABLE ht;

CREATE TABLE ht AS
  SELECT 1 id, 'A' c1, 'B' c2 FROM DUAL
  UNION
  SELECT 1 id, 'C' c1, 'A' c2 FROM DUAL
  UNION
  SELECT 3 id, 'B' c1, 'D' c2 FROM DUAL
  UNION
  SELECT 4 id, 'E' c1, 'B' c2 FROM DUAL
  UNION
  SELECT 4 id, 'F' c1, 'E' c2 FROM DUAL
  UNION
  SELECT 6 id, 'B' c1, 'G' c2 FROM DUAL
  UNION
  SELECT 7 id, 'X' c1, 'Y' c2 FROM DUAL
  UNION
  SELECT 8 id, 'Y' c1, 'Z' c2 FROM DUAL;

SELECT       CONNECT_BY_ROOT c1 c1, c2, id, CONNECT_BY_ROOT id id
FROM         ht a
WHERE        CONNECT_BY_ISLEAF = 1
CONNECT BY   PRIOR c2 = c1
START WITH   c1 NOT IN (SELECT   c2
                        FROM     ht);

DESIRED
C1	C2
C	D
F	G
X	Z

ACTUAL
C1	C2
C	D
C	G
F	D
F	G
X	Z

WITH a AS
       (SELECT       CONNECT_BY_ROOT c1 c1, c2, CONNECT_BY_ISLEAF leaf, LEVEL lev, MAX(LEVEL) OVER (PARTITION BY c2) mlev, id, MIN(id) OVER (PARTITION BY CONNECT_BY_ROOT c1) mid
        FROM         ht a
        WHERE        CONNECT_BY_ISLEAF = 1
        CONNECT BY   PRIOR c2 = c1 AND PRIOR id <= id)
SELECT   c1, c2
FROM     a
WHERE    lev = mlev AND id = mid;