queens.a68
1 CO
2
3 @section Synopsis
4
5 'N' queens in classic backtracker.
6
7 CO
8
9 BEGIN
10
11 INT rank = 8;
12 printf (($"Rank="g(0)l$, rank));
13 [1 : rank] CCOL column, INT sols found := 0;
14
15 PROC place from = (CROW this row) VOID:
16 IF this row > rank
17 THEN sols found +:= 1;
18 FOR row TO rank
19 DO print(("abcdefghijklmnopqrstuvwxyz"[row], whole(column[row], 0), " "))
20 OD;
21 print ((sols found MOD 4 = 0 | new line | " "))
22 ELSE FOR i TO rank
23 DO IF BOOL safe := TRUE;
24 FOR j TO this row - 1
25 WHILE safe
26 DO safe := safe AND NOT (column[j] = i ORF (column[j] - i = this row - j ORF column[j] - i = j - this row))
27 OD;
28 safe
29 THEN column[this row] := i;
30 place from (this row + 1)
31 FI
32 OD
33 FI;
34
35 place from (1);
36
37 MODE CROW = INT, CCOL = INT;
38
39 SKIP
40
41 END
