Sudoku Game with C++ - day 3

I made 9 x 9 Matrix with all numbers allocated randomly. Then, I checked if all columns, rows, and 9 3by3 matrices have no repeated numbers.

However, this algorithm takes so much time to generate correct sudoku matrix. At first, I didn't realize how bad this is. So, I made 2 by 2 matrix with same algorithm, But It took so long time to find correct Matrix even it has only 4 numbers!!

Now, I think that it is because I made whole matrix and then judged if it is well made. So I would make another algorithm.