hi, Im coding a sodoku program base on backtracking, but somehow it did not work as what I expected, can help me figure out what's the problem?

```
sodoku.c
#include <stdio.h>
#include <stdlib.h>
#define true 1
#define false 0
void printGrid(int board[]) {
int i,j;
if (board==NULL){
printf("No Solution\n");
return;
}
for (i=0;i<9;i++){
for (j=0;j<9;j++){
printf ("%d",board[i*9+j]);
if (j%3==2)
printf (" ");
}
printf ("\n");
if (i%3==2)
printf ("\n");
}
}
int checkDigit(int board[], int digit, int n) {
int i,j;
int ROW;
int COL;
ROW=n/9;
COL=n%9;
if (ROW<0 || COL<0 || ROW>8 || COL >8 || digit <1 || digit >9 )
return false;
if (board[ROW*9+COL]!=0)
return false;
//scan horizontally and vertically
for (i=0;i<9;i++){
if (board[i*9+COL]==digit || board[ROW*9+i]==digit)
return false;
}
//scan within the square
for (j=((ROW/3)*3);j<(((ROW/3)*3)+3);j++){
for (i=((COL/3)*3);i<((COL/3)*3+3);i++){
if (board[j*9+ROW]==digit)
return false;
}
}
return true;
}
int solveGrid(int board[], int n) {
int i,j,k;
i=n/9;
j=n%9;
int solved=1;
for (n=0;n<81;n++){
if (board[n]==0){
solved=0;
}
else
solved=1;
}
if (solved)
return;
for (i=0;i<9;i++){
for (j=0;j<9;j++){
if (board[i*9+j]==0){
for (k=1;k<=9;k++){
if (checkDigit(board,k,i*9+j)){
board[i*9+j]=k;
solveGrid(board,i*9+j+1);
}
}
}
}
}
}
main.c
#include <stdio.h>
#include <stdlib.h>
#define ROW 9
#define COL 9
int main(int argc, char** argv) {
/* the sudoku grid is represented by this 1D array */
int board[ROW * COL];
char filename[100];
int i, j, input;
FILE *fp;
printf("Enter data file name: ");
scanf("%s", filename);
fp = fopen(filename, "r");
if (fp == NULL) {
printf("Cannot open data file\n");
exit(0);
}
for (i = 0; i < 9; i++) {
for (j = 0; j < 9; j++) {
fscanf(fp, "%d", &input);
board[i * ROW + j] = input;
}
}
fclose(fp);
// the solver starts from position 0
if (solveGrid(board, 0)) {
printf("Solved.\n");
printGrid(board);
}
else
printf("No Solution.");
return (EXIT_SUCCESS);
}
```