N-Queens Problem

queens(N):-

length(List,N),

List in 1..N,

constrain_queens(List),

labeling(List),

write(List),nl.

constrain_queens([]).

constrain_queens([X|Y]):-

safe(X,Y,1),

constrain_queens(Y).

safe(_,[],_).

safe(X,[Y|T],K):-

noattack(X,Y,K),

K1 is K+1,

safe(X,T,K1).

noattack(X,Y,K):-

X #\= Y,

X+K #\= Y,

X-K #\= Y.

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