sudoku.c

#include <ctype.h>
#include <stdio.h>
unsigned char IsSafe(const unsigned char *const board, const unsigned char pos,
                     const unsigned char num) {
  const unsigned char row = pos / 9, col = pos % 9, row_start = row / 3 * 27,
                      col_start = col / 3 * 3;
  unsigned char i;
  for (i = 0; i < 9; ++i)
    if (board[row * 9 + i] == num || board[i * 9 + col] == num ||
        board[row_start + i % 3 * 9 + col_start + i / 3] == num)
      return 0;
  return 1;
}
unsigned char Solve(unsigned char *board) {
  unsigned char i;
  unsigned char *p = board;
  for (i = 0; i < 81; ++i, ++p) {
    if (*p == 0) {
      unsigned char num;
      for (num = 1; num <= 9; ++num) {
        if (IsSafe(board, i, num)) {
          *p = num;
          if (Solve(board)) return 1;
          *p = 0;
        }
      }
      return 0;
    }
  }
  return 1;
}
void PrintSolution(unsigned char *board) {
  if (Solve(board)) {
    unsigned char i;
    for (i = 0; i < 81; ++i) {
      printf("%u", *board++);
      if (i % 9 == 8) putchar('\n');
    }
  } else
    puts("No solution");
}
unsigned char ReadInput(unsigned char *arr) {
  const unsigned char *const end = arr + 81;
  int c;
  while (arr < end) {
    c = getchar();
    if (isdigit(c))
      *arr++ = c - '0';
    else if (c == '.')
      *arr++ = 0;
    else
      return 0;
  }
  while ((c = getchar()) != '\n' && c != EOF)
    ;
  return 1;
}
int main() {
  unsigned char board[81];
  while (1)
    if (ReadInput(board)) PrintSolution(board);
  return 0;
}