You have to place N queens on an N-by-N chessboard so that no two queens are on the same row, column or diagonal.
/* queens(N, Queens) is true if Queens is a placement that solves the N */ /* queens problem, represented as a permutation of the list of integers */ /* [1, 2, ..., N]. */ queens(N, Queens):- N > 0, integers(1, N, Rows), queens_1(Rows, N, , , , Queens). queens_1(, 0, Queens, _, _, Queens). queens_1(Rows, Col, A, B, C, Queens):- remove(Row, Rows, Rows1), /* All squares on the same NW-SE diagonal have the same value of Row+Col */ RowPlusCol is Row + Col, \+ member(RowPlusCol, B), /* All squares on the same SW-NE diagonal have the same value of Row-Col */ RowMinusCol is Row - Col, \+ member(RowMinusCol, C), Col1 is Col - 1, queens_1(Rows1, Col1, [Row|A], [RowPlusCol|B], [RowMinusCol|C], Queens). /* integers(M, N, Is) is true if Is is the list of integers from M to N */ /* inclusive. */ integers(N, N, [N]):-!. integers(I, N, [I|Is]):-I < N, I1 is I + 1, integers(I1, N, Is).
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