! 例題 8-12  「ハノイの塔」の解 ［再帰的手続きの定番 ］
PROGRAM ex8_12

  IMPLICIT NONE
  INTEGER :: n
  CHARACTER ( LEN = 1 ) :: a = 'A', b = 'B', c = 'C'

  WRITE ( *, '( A )', ADVANCE = 'NO' ) '最初の輪の個数は？ '
  READ *, n

  CALL hanoi( n, a, b, c )

END PROGRAM ex8_12
!
RECURSIVE SUBROUTINE hanoi( i, x, y, z )

  IMPLICIT NONE
  INTEGER, INTENT ( IN ) :: i
  INTEGER, SAVE :: count = 0
  CHARACTER ( LEN = 1 ), INTENT ( IN ) :: x, y, z

  IF ( i == 0 ) RETURN

  CALL hanoi( i - 1, x, z, y )         !   手順 (1)
  CALL output
  CALL hanoi( i - 1, z, y, x )         !   手順 (3)

CONTAINS

  SUBROUTINE output
    count = count + 1
    PRINT '( 1X, I3, ") 第", I1, "番目の輪を ", A1, " から ", &
             & A1, " へ移動する"  )', count, i, x, y
  END SUBROUTINE output

END SUBROUTINE hanoi