Consider a 5 x 5 version of a chessboard with one player having five queens and the other player having three queens. There are no other pieces. Place the queens on the board so that neither player's queens can capture one of his or her opponent's queens.

*** For the purpose of simplicity, assume the white side has the greater number of queens.

DIM used(24)
OPEN "queenplc.txt" FOR OUTPUT AS #2
FOR a = 0 TO 24
 used(a) = 1
 arow = a \ 5: acol = a MOD 5
 bd(arow, acol) = 1
FOR b = a + 1 TO 24
 IF used(b) = 0 THEN
   used(b) = 1
   brow = b \ 5: bcol = b MOD 5
   bd(brow, bcol) = 1
FOR c = b + 1 TO 24
 IF used(c) = 0 THEN
   used(c) = 1
   crow = c \ 5: ccol = c MOD 5
   bd(crow, ccol) = 1
FOR d = c + 1 TO 24
 IF used(d) = 0 THEN
   used(d) = 1
   drow = d \ 5: dcol = d MOD 5
   bd(drow, dcol) = 1
FOR e = d + 1 TO 24
 IF used(e) = 0 THEN
   used(e) = 1
   erow = e \ 5: ecol = e MOD 5
   bd(erow, ecol) = 1

   safect = 0
   FOR r = 0 TO 4
     FOR cl = 0 TO 4
       safe = 1
       IF arow = r OR brow = r OR crow = r OR drow = r OR erow = r THEN safe = 0
       IF acol = cl OR bcol = cl OR ccol = cl OR dcol = cl OR ecol = cl THEN safe = 0
       IF ABS(arow - r) = ABS(acol - cl) THEN safe = 0
       IF ABS(brow - r) = ABS(bcol - cl) THEN safe = 0
       IF ABS(crow - r) = ABS(ccol - cl) THEN safe = 0
       IF ABS(drow - r) = ABS(dcol - cl) THEN safe = 0
       IF ABS(erow - r) = ABS(ecol - cl) THEN safe = 0
       IF safe THEN safect = safect + 1: safer(safect) = r: safec(safect) = cl
     NEXT
   NEXT
   IF safect > 2 THEN
      FOR r = 0 TO 4
       FOR cl = 0 TO 4
        flag = 0
          FOR r2 = 1 TO safect
           IF r = safer(r2) AND cl = safec(r2) THEN PRINT #2, " x"; : flag = 1
          NEXT
          IF flag = 0 THEN IF bd(r, cl) THEN PRINT #2, " *"; :  ELSE PRINT #2, " .";
       NEXT: PRINT #2,
      NEXT: PRINT #2,
      solct = solct + 1
   END IF

   bd(erow, ecol) = 0
   used(e) = 0
 END IF
NEXT e
   bd(drow, dcol) = 0
   used(d) = 0
 END IF
NEXT d
   bd(crow, ccol) = 0
   used(c) = 0
 END IF
NEXT c
   bd(brow, bcol) = 0
   used(b) = 0
 END IF
NEXT b
 bd(arow, acol) = 0
 used(a) = 0
NEXT a

PRINT solct

CLOSE

finds all 8 versions (rotations and reflections) of the solution, using * and x to represent the two colors:

 . * * . .
 . * . . *
 . . . . *
 x . . . .
 x . . x .

 . * * . .
 . . . . x
 * . . . .
 * * . . .
 . . . x x

 . . * * .
 * . . * .
 * . . . .
 . . . . x
 . x . . x

 . . * * .
 x . . . .
 . . . . *
 . . . * *
 x x . . .

 . . . x x
 * * . . .
 * . . . .
 . . . . x
 . * * . .

 x x . . .
 . . . * *
 . . . . *
 x . . . .
 . . * * .

 . x . . x
 . . . . x
 * . . . .
 * . . * .
 . . * * .

 x . . x .
 x . . . .
 . . . . *
 . * . . *
 . * * . .

The program can easily be modified to answer this question:

If six white queens are appropriately placed on a 6x6 board, how many black queens can be accommodated in the same manner as in the current puzzle?

Posted by Charlie on 2013-07-05 19:25:35