C code to solve any Sudoku puzzle

sudoku_solver.c

#include<stdio.h>

// A sample puzzle
int sudoku[9][9] =    { {0,0,0,4,0,0,0,9,0},
                        {0,3,0,0,5,0,0,1,8},
                        {0,0,0,0,0,0,6,0,7},
                        {4,2,0,1,0,3,0,0,6},
                        {0,9,0,0,0,0,0,2,0},
                        {7,0,0,2,0,8,0,4,1},
                        {6,0,4,0,0,0,0,0,0},
                        {9,8,0,0,2,0,0,6,0},
                        {0,5,0,0,0,9,0,0,0} };

// Print Sudoku puzzle on the screen
void printSudoku (void) {
        int x;
        int y;
        for (x = 0; x < 9; x++) {
                for (y = 0; y < 9; y++) {
                        if (y%3 == 0) printf ("+");
                        if (x%3 == 0) {
                                printf("+===");
                        } else {
                                printf("+---");
                        }
                }
                printf ("++\n");
                for (y = 0; y < 9; y++) {
                        if (y%3 == 0) printf ("|");
                        printf ("| %d ", sudoku[x][y]);
                }
                printf ("||\n");
        }
        for (y = 0; y < 9; y++) {
                if (y%3 == 0) printf ("+");
                printf("+===");
        }
        printf ("++\n");
}

// Check if it is possible to put a number "n" at given cell (x,y)
int isPossible (int x, int y, int n) {
        int i;
        int j;
        int bx;
        int by;
        // Empty cell?
        if (sudoku[x][y] != 0) {
                return 0;
        }
        // Check row
        for (i = 0; i < 9; i++) {
                if (sudoku[x][i] == n) {
                        return 0;
                }
        }
        // Check column
        for (j = 0; j < 9; j++) {
                if (sudoku[j][y] == n) {
                        return 0;
                }
        }
        // Check in corresponding 3x3 box
        bx = (x/3) * 3;
        by = (y/3) * 3;
        for (i = 0; i < 3; i++) {
                for (j = 0; j < 3; j++) {
                        if (sudoku[bx + i][by + j] == n) {
                                return 0;
                        }
                }
        }
        // Not found anywhere?
        return 1;
}

// Try to solve given sodoku puzzle and find all possible solutions (if any).
// It is a recursive function which returns when
// 1. There are no empty cells i.e. solved.
// 2. There is a conflict (i.e. cannot put any number) in a cell.
void solve (void) {
        int x;
        int y;
        int n;
        for (x = 0; x < 9; x++) {
                for (y = 0; y < 9; y++) {
                        if (sudoku[x][y] == 0) {
                                for (n = 1; n < 10; n++) {
                                        if (isPossible (x, y, n) == 1) {
                                                // Assume n is correct value and try to solve the puzzle
                                                sudoku[x][y] = n;
                                                solve ();
                                                // We might be here because of some conflict during previous call
                                                // Clear the value to try next one
                                                sudoku[x][y] = 0;
                                        }
                                }
                                // No match. Conflict!!
                                return;
                        }
                }
        }
        // We are here because one of the recursive call has solved the puzzle.
        // Print the solution here.
        printSudoku ();
        // Below return will be considered as a conflict by caller
        // and it will proceed in search of anther solutions (if any)
        // so we will lose this solution. If required, save the solution in some other buffer.
        return;
}

// Get an unsolved Sudoku puzzle from user.
void getSudoku (void) {
        int x;
        int y;
        int n;
        printf ("Enter Sudoku puzzle cell by cell:\n");
        for (x = 0; x < 9; x++) {
                for (y = 0; y < 9; y++) {
                        printf ("Enter element (%d,%d):", x, y);
                        scanf ("%1d", &n);
                        sudoku[x][y] = n;
                }
        }
}

// Main function
int main (int argc, char** argv) {
        getSudoku (); // Remove this line if you just want to solve sample puzzle.
        printf ("Unsolved Sudoku puzzle:\n");
        printSudoku ();
        printf ("Solution:\n");
        solve ();
        return 0;
}