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Task/N-queens-problem/Liberty-BASIC/n-queens-problem.basic

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+'N queens
+'>10 would not work due to way permutations used
+'anyway, 10 doesn't fit in memory
+Input "Input N for N queens puzzle (4..9) ";N
+if N<4 or N>9 then print "N out of range - quitting": end
+
+ABC$= " "
+dash$ = ""
+for i = 0 to N-1
+    ABC$=ABC$+" "+chr$(asc("a")+i)
+    dash$ = dash$+"--"
+next
+
+dim q(N)
+t0=time$("ms")
+
+fact = 1
+for i = 1 to N
+    fact = fact*i
+next
+
+dim anagram$(fact)
+global nPerms
+print "Filling permutations array"
+t0=time$("ms")
+    res$=permutation$("", left$("0123456789", N))
+t1=time$("ms")
+print "Created all possible permutations ";t1-t0
+
+t0=time$("ms")
+'actually fact = nPerms
+for k=1 to nPerms
+    for i=0 to N-1
+        q(i)=val(mid$(anagram$(k),i+1,1))
+        'print q(i);
+    next
+    'print
+
+    fail = 0
+    for i=0 to N-1
+        for j=i+1 to N-1
+            'check rows are different
+            if q(i)=q(j) then fail = 1: exit for
+            'check diagonals are different
+            if i+q(i)=j+q(j) then fail = 1: exit for
+            'check other diagonals are different
+            if i-q(i)=j-q(j) then fail = 1: exit for
+        next
+        if fail then exit for
+    next
+
+    if not(fail) then
+        num=num+1
+        print " ";dash$
+            for i=0 to N-1
+                print N-i; space$(2*q(i));" *"
+            next
+        print " ";dash$
+        print ABC$
+    end if
+
+next
+
+t1=time$("ms")
+print "Time taken ";t1-t0
+print "Number of solutions ";num
+
+'----------------------------------
+'from
+'http://babek.info/libertybasicfiles/lbnews/nl124/wordgames.htm
+'Programming a Word Game by Janet Terra,
+'The Liberty Basic Newsletter - Issue #124 - September 2004
+Function permutation$(pre$, post$)
+'Note the variable nPerms must first be stated as a global variable.
+    lgth = Len(post$)
+    If lgth < 2 Then
+        nPerms = nPerms + 1
+        anagram$(nPerms) = pre$;post$
+    Else
+        For i = 1 To lgth
+            tmp$=permutation$(pre$+Mid$(post$,i,1),Left$(post$,i-1)+Right$(post$,lgth-i))
+        Next i
+    End If
+End Function