C SUDOKU solver in 558 chars
============================

sudoku.c is a tiny, brute force sudoku solver.

SOURCE:
=======

#include <stdio.h>

int s[81], m[82], k, i;

main() {
for (i=0;i<81;++i) {
scanf("%d",&s[i]);
m[i]=s[i];
}
for(;;) {
while (m[k]) k++;
if (k>=81) {
  for (i=0;i<81;++i) {
printf("%d ",s[i]);
if (i%9>7) printf("\n");
  }
  return;
}
if (++s[k]>9) {
  s[k--]=0;
  for(;m[k];)
if (k--==0) return;
} else {
  for (i=0;i<81;++i) {
if (i==k || s[i]==0) continue;
if (i%9==k%9 || i/9==k/9 ||
  (i/3)%3==(k/3)%3 && i/3/9==k/3/9)
if (s[i]==s[k]) break;
  }
  if (i>=81)
k++;
}
}
}


COMPACT VERSION (295 chars)
===========================

This (sudoku2.c) is a compacted version of the previous code.
Can you make it smaller? Let me know!

#define F for(;i<81
s[81],m[82],k,i,e;main(){F;++i)scanf("%d",s+i),m[i]=s[i];for(;!e;
){for(i=0;m[k];)k++;if(e=k>80)F;printf("%d%c",s[i++],i%9>7?10:32));else
if(++s[k]>9)for(s[k--]=0;m[k];e=!k--);else{F&&(i==k||s[i]==0||s[i]-s[k]
||i%9-k%9&&i/9-k/9&&(i/3%3-k/3%3||i/3/9-k/3/9));++i);k+=i/81;}}}

COMPILATION:
============

$ gcc -o sudoku sudoku.c

The files grid<n> are some sample input grids.
The file grid9 is a particularly difficul input.
On my PC it take more 16 minutes to solve this.

USAGE:
======

$ cat grid1
0 5 0 0 6 0 0 0 1
0 0 4 8 0 0 0 7 0
8 0 0 0 0 0 0 5 2
2 0 0 0 5 7 0 3 0
0 0 0 0 0 0 0 0 0
0 3 0 6 9 0 0 0 5
7 9 0 0 0 0 0 0 8
0 1 0 0 0 6 5 0 0
5 0 0 0 3 0 0 6 0

$ ./sudoku < grid1
9 5 3 7 6 2 8 4 1
6 2 4 8 1 5 9 7 3
8 7 1 3 4 9 6 5 2
2 8 9 4 5 7 1 3 6
1 6 5 2 8 3 4 9 7
4 3 7 6 9 1 2 8 5
7 9 6 5 2 4 3 1 8
3 1 8 9 7 6 5 2 4
5 4 2 1 3 8 7 6 9

HISTORY
=======

31.03.2009:

I was amazed by the OCaml Sudoku solver by Marco Maggesi:

http://web.math.unifi.it/users/maggesi/repos/ocaml-sudoku-solver/

So I decided I should learn OCaml! But... I failed. So, I wrote the
Sudoku solver in my preferred computer language.