A buggy sudoku solver.

sudoku.c

#include <stdio.h>

int sudoku[9][9];
int row_count[9][10];
int col_count[9][10];
int box_count[3][3][10];

int check(int r, int c){
  if(row_count[r][sudoku[r][c]] != 1)
    return 0;
  if(col_count[c][sudoku[r][c]] != 1)
    return 0;
  if(box_count[r/3][c/3][sudoku[r][c]] != 1)
    return 0;
  return 1;
}

void set_count(int r, int c){
  ++row_count[r][sudoku[r][c]];
  ++col_count[c][sudoku[r][c]];
  ++box_count[r/3][c/3][sudoku[r][c]];
}

void reset_count(int r, int c){
  --row_count[r][sudoku[r][c]];
  --col_count[c][sudoku[r][c]];
  --box_count[r/3][c/3][sudoku[r][c]];
}

int solver(int r, int c){
  if(r == 9) /* all cells are filled */
    return 1;
  if(sudoku[r][c]){ /* the cell is prefilled */
    if(c < 8) return solver(r, c + 1);
    else      return solver(r + 1, 0);
  }
  /* try all digits to fill the cell */
  for(sudoku[r][c]=1; sudoku[r][c]<=9; ++sudoku[r][c]){
    set_count(r, c);
    if(check(r, c)){
      if(c < 8){
        if(solver(r, c + 1)) { 
          return 1;
        }
      }else{
        if(solver(r + 1, 0)) {
          return 1;
        }
      }
    }
    reset_count(r, c);
  }
  return 0;
}

int main(){
  int r, c;

  for(r = 0; r < 9; ++r){
    for(c = 0; c < 9; ++c){
      scanf("%d", &sudoku[r][c]);
      set_count(r, c);
    }
  }
  if(solver(0, 0)){
    for(r = 0; r < 9; ++r){
      for(c = 0; c < 8; ++c)
        printf("%d ", sudoku[r][c]);
      printf("%d\n", sudoku[r][8]);
    }
  }else
    printf("no solution\n");

  return 0;
}