Data update

Task/N-queens-problem/Pascal/n-queens-problem-1.pas

@@ -1,81 +1,34 @@
-program queens;
+program eightqueens(output);
+var i: integer;
+    a: array [1..8] of boolean; { a[j]: no queen in row j }
+    b: array [2..16] of boolean; { b[k]: no queen in kth diagonal down-left }
+    c: array [-7..7] of boolean; { c[k]: no queen in kth diagonal down-right }
+    x: array [1..8] of integer; { x[i]: position of queen in column i }
+procedure print;
+    var k: integer;
+begin
+    for k := 1 to 8 do write(x[k]: 4);
+    writeln
+end { print } ;
 
-const l=16;
-
-var i,j,k,m,n,p,q,r,y,z: integer;
-    a,s: array[1..l] of integer;
-    u: array[1..4*l-2] of integer;
-
-label L3,L4,L5,L6,L7,L8,L9,L10;
+procedure try(i: integer);
+    var j: integer;
+begin
+    for j := 1 to 8 do
+        if a[j] and b[i+j] and c[i-j] then
+        begin
+            { place queen }
+            x[i] := j;
+            a[j] := false; b[i+j] := false; c[i-j] := false;
+            if i < 8 then try(i+1) else print;
+            { remove queen }
+            a[j] := true; b[i+j] := true; c[i-j] := true
+        end
+end { try } ;
 
 begin
-   for i:=1 to l do a[i]:=i;
-   for i:=1 to 4*l-2 do u[i]:=0;
-   for n:=1 to l do
-   begin
-      m:=0;
-      i:=1;
-      r:=2*n-1;
-      goto L4;
-L3:
-      s[i]:=j;
-      u[p]:=1;
-      u[q+r]:=1;
-      i:=i+1;
-L4:
-      if i>n then goto L8;
-      j:=i;
-L5:
-      z:=a[i];
-      y:=a[j];
-      p:=i-y+n;
-      q:=i+y-1;
-      a[i]:=y;
-      a[j]:=z;
-      if (u[p]=0) and (u[q+r]=0) then goto L3;
-L6:
-      j:=j+1;
-      if j<=n then goto L5;
-L7:
-      j:=j-1;
-      if j=i then goto L9;
-      z:=a[i];
-      a[i]:=a[j];
-      a[j]:=z;
-      goto L7;
-L8:
-      m:=m+1;
-      { uncomment the following to print solutions }
-      { write(n,' ',m,':');
-      for k:=1 to n do write(' ',a[k]);
-      writeln; }
-L9:
-      i:=i-1;
-      if i=0 then goto L10;
-      p:=i-a[i]+n;
-      q:=i+a[i]-1;
-      j:=s[i];
-      u[p]:=0;
-      u[q+r]:=0;
-      goto L6;
-L10:
-      writeln(n,' ',m);
-   end;
-end.
-
-{ 1 1
-  2 0
-  3 0
-  4 2
-  5 10
-  6 4
-  7 40
-  8 92
-  9 352
- 10 724
- 11 2680
- 12 14200
- 13 73712
- 14 365596
- 15 2279184
- 16 14772512 }
+    for i := 1 to 8 do a[i] := true;
+    for i := 2 to 16 do b[i] := true;
+    for i := -7 to 7 do c[i] := true;
+    try(1)
+end .