sudoku.c

#include <stdio.h>

int sudoku[9][9];

int read() {
    for (int y = 0; y < 9; y++) {
        for (int x = 0; x < 9; x++) {
            scanf("%d", &sudoku[x][y]);
        }
    }
}

int print() {
    for (int y = 0; y < 9; y++) {
        for (int x = 0; x < 9; x++) {
            if (sudoku[x][y])
                printf("%d ", sudoku[x][y]);
            else
                printf("  ");
            if (x == 2 || x == 5)
                printf("| ");
        }
        printf("\n");
        if (y == 2 || y == 5)
            printf("------+-------+------\n");
    }
}

int valid(int i, int j) {
    for (int x = 0; x < 9; x++)
        if (x != i && sudoku[i][j] == sudoku[x][j])
            return 0;
    for (int y = 0; y < 9; y++)
        if (y != j && sudoku[i][j] == sudoku[i][y])
            return 0;
    int cx = i / 3;
    int cy = j / 3;
    for (int x = 0; x < 3; x++)
        for (int y = 0; y < 3; y++)
            if (3*cx+x != i && 3*cy+y != j && sudoku[i][j] == sudoku[3*cx+x][3*cy+y])
                return 0;
    return 1;
}

int backtrack(int i, int j) {
    if (i == 9 && j == 8)
        return 1;
    if (i == 9)
        return backtrack(0, j+1);
    for (int n = 1; n <= 9; n++) {
        sudoku[i][j] = n;
        if (valid(i, j) && backtrack(i+1, j))
            return 1;
        sudoku[i][j] = 0;
    }
    return 0;
}

int main() {
    read();
    print();
    printf("\n");
    backtrack(0, 0);
    print();
}

sudoku.txt

0 0 4 3 7 0 0 9 2
0 2 8 0 0 1 0 0 7
0 3 5 0 8 0 6 0 1
1 0 2 0 3 0 5 7 8
4 7 0 5 9 8 0 0 3
0 0 0 1 0 0 9 6 4
0 0 9 8 0 6 7 3 5
0 0 7 0 4 3 0 0 6
3 6 0 0 0 2 0 8 0