sudoku.c

#include <stdio.h>

const int GRID_SIZE = 9;

void print(int grid[GRID_SIZE][GRID_SIZE]) {
	for (int i=0;i<GRID_SIZE;i++) {
		for (int j=0;j<GRID_SIZE;j++) {
			printf("%d", grid[i][j]);
			if (j < GRID_SIZE-1) {
				printf(" ");
			}
		}
		printf("\n");
	}
}

int fits(int grid[GRID_SIZE][GRID_SIZE], int row, int col, int n)
{
	int rs = (row/3) * 3;
	int cs = (col/3) * 3;

	for(int i=0; i<GRID_SIZE; i++) {
		if (grid[row][i] == n) {
			return 0;
		}
		if (grid[i][col] == n) {
			return 0;
		}
		if (grid[rs+(i%3)][cs+(i/3)] == n) {
			return 0;
		}
	}
	return 1;
}

int rsolve(int grid[GRID_SIZE][GRID_SIZE], int row, int col) {
	if (row == GRID_SIZE && col == GRID_SIZE) {
		return 1;
	}

	if (grid[row][col] != 0) {
		if ( col < GRID_SIZE-1) {
			return rsolve(grid, row, col+1);
		} else if (row < GRID_SIZE-1) {
			return rsolve(grid, row+1, 0);
		}
		return 1;
	}
	
	for (int n=0; n<GRID_SIZE; n++) {
		if (fits(grid, row, col, n+1)) {
			grid[row][col] = n+1;
			if (col < GRID_SIZE-1) {
				if (rsolve(grid, row, col+1)) {
					return 1;
				}
				grid[row][col] = 0;
			}
			else if (row < GRID_SIZE-1) {
				if (rsolve(grid, row+1, 0)) {
					return 1;
				}
				grid[row][col] = 0;
			}
			else {
				return 1;
			}
		}
	}
	return 0;
}

void solve(int grid[GRID_SIZE][GRID_SIZE]) {
	rsolve(grid, 0, 0);
}

int main(int argc, char *argv[]) {
	if (argc != GRID_SIZE+1) {
		return 1;
	}

	int grid[GRID_SIZE][GRID_SIZE];

	for (int i=0; i<GRID_SIZE; i++) {
		for (int j=0; j<GRID_SIZE; j++) {
			char c = argv[i+1][j];
			grid[i][j] = c == '.' ? 0 : c - '0';
		}
	}

	solve(grid);
	print(grid);

	return 0;	
}