tasks a-s

Task/N-queens-problem/Prolog/n-queens-problem-2.pro

@@ -0,0 +1,28 @@
+solution(Queens) :-
+ permutation([1,2,3,4,5,6,7,8], Queens),
+ safe(Queens).
+
+permutation([],[]).
+
+permutation([Head|Tail],PermList) :-
+ permutation(Tail,PermTail),
+ del(Head,PermList,PermTail).
+
+del(Item,[Item|List],List).
+
+del(Item,[First|List],[First|List1]) :-
+ del(Item,List,List1).
+
+safe([]).
+
+safe([Queen|Others]) :-
+ safe(Others),
+ noattack(Queen,Others,1).
+
+noattack(_,[],_).
+
+noattack(Y,[Y1|Ylist],Xdist) :-
+ Y1-Y=\=Xdist,
+ Y-Y1=\=Xdist,
+ Dist1 is Xdist + 1,
+ noattack(Y,Ylist,Dist1).