The n queen problem implemented by bit operations

C language is the first language I learned in my professional courses in my freshman year, almost five years ago. I recently bought a copy of The Plauger bull <The standard C library>. I simply flipped it over and thought I was just like a slot... In the past, the understanding of C was just superficial. I do not know that this extremely good cross-platform portable compilation language has such a wonderful implementation.
I sorted out the document a few days ago and found this section of the N queen algorithm implemented by C bit. It took me half a day to figure it out .. As we all know, the general solution of Queen n is backtracking. We need to use a two-dimensional array to represent the chessboard and perform a search by row (Column). We need to judge each current solution, if it is successful, it continues; if it fails, it goes back.
The Code implemented with bit is extremely concise. However, it only uses a one-dimensional array and searches by row to normalize the restrictions on the column and object line.
Interested friends can look at: html "> http://jsomers.com/nqueen_demo/nqueens.html author method is similar to the following method.
1 # include <stdio. h>
2 # include <stdlib. h>
3 # include <time. h>
4
5 long int sum = 0, upperlim = 1;
6
7 void test (long row, long ld, long rd)
8 {
9 // printf ("% ld", row, ld, rd );
10 // printf ("% ld", row );
11 // printf ("% ld", upperlim );
12 if (row! = Upperlim)
13 {
14 long pos = upperlim &~ (Row | ld | rd );
15 // printf ("% ld", pos );
16 while (pos! = 0)
17 {
18 // printf ("% ld", pos );
19 long p = pos &-pos;
20 pos = pos-p; // obtain the rightmost column of the Queen.
21 test (row + p, (ld + p) <1, (rd + p)> 1 );
22}
23 return;
24}
25 else
26 {
27 sum ++;
28 return;
29}
30}
31
32 int main (int argc, char * argv [])
33 {
34 time_t tm;
35 int n = 16;
36 if (argc! = 1) n = atoi (argv [1]);
37 tm = time (0 );
38 if (n <1) | (n> 32 ))
39 {
40 printf ("only Queen issues between 1-32 can be calculated ");
41 exit (-1 );
42}
43 printf ("% d ", n );
44 printf ("% ld", upperlim );
45 upperlim = (upperlim <n)-1; // all bits are 1
46 printf ("% ld", upperlim );
47 test (0, 0 );
48 printf ("% ld in total, calculation time % d seconds", sum, (int) (time (0)-tm ));
49 system ("pause ");
50 return 0;
51}