16*16 Sudoku solver using backtracking in c - c

Question

I have coded a 16*16 sudoku solver using the backtracking algorithm in c. The program takes input from the user using a text file. The unfilled positions in the matrix are filled with a 0. Works perfectly fine.
I want to further optimize the code for time and space complexity. Please suggest some methods or ways to optimize the code.
#include <stdio.h>
#include<time.h>
#define N 16
int isAvailable(int sudoku[16][16], int row, int col, int num)
{
int i, j;
for(i=0; i<16; i++){
if( (sudoku[row][i] == num) || ( sudoku[i][col] == num ) )
return 0;
}
int rowStart = (row/4) * 4;
int colStart = (col/4) * 4;
for(i=rowStart; i<(rowStart+4); ++i)
{
for(j=colStart; j<(colStart+4); ++j)
{
if( sudoku[i][j] == num )
return 0;
}
}
return 1;
}
int fillsudoku(int sudoku[16][16], int row, int col)
{
int i;
if( row<16 && col<16 )
{
if( sudoku[row][col] != 0 )
{
if( (col+1)<16 )
return fillsudoku(sudoku, row, col+1);
else if( (row+1)<16 )
return fillsudoku(sudoku, row+1, 0);
else
return 1;
}
else
{
for(i=0; i<16; ++i)
{
if( isAvailable(sudoku, row, col, i+1) )
{
sudoku[row][col] = i+1;
if( (col+1)<16 )
{
if( fillsudoku(sudoku, row, col+1) )
return 1;
else
sudoku[row][col] = 0;
}
else if( (row+1)<16 )
{
if( fillsudoku(sudoku, row+1, 0) )
return 1;
else
sudoku[row][col] = 0;
}
else
return 1;
}
}
}
return 0;
}
else
{
return 1;
}
}
int main()
{
clock_t tic=clock();
int a,i,j;
int sudoku[N][N];
FILE *f;
f=fopen("s16.txt","r");
for(i=0;i<N;i++){
for(j=0;j<N;j++)
{
fscanf(f,"%d",&sudoku[i][j]);
}
}
if( fillsudoku(sudoku, 0, 0) )
{
for(i=0; i<16; ++i)
{
for(j=0; j<16; ++j){
printf("%d ", sudoku[i][j]);
if(sudoku[i][j]<10){
printf(" ");
}
if(j%4==3){
printf("|");
}
}
printf("\n");
if(i%4==3){
printf("----------------------------------------------------");
}
printf("\n");
}
}
else
{
printf("\n\nNO SOLUTION\n\n");
}
clock_t toc=clock();
printf("\nTotal time elasped is %f", (double)(toc - tic) / CLOCKS_PER_SEC);
return 0;
fclose(f);
}