Sudoku is a logic-based, combinatorial number-placement puzzle. In classic sudoku, the objective is to fill a 9×9 grid with digits so that each column, each row, and each of the nine 3×3 subgrids that compose the grid contains all of the digits from 1 to 9.

How does Sudoku Solver Work?
- Fill the blank with number 1 - 9
- Every fill the blank the position and value store for backtrack
- If the blank not suitable with number 1 - 9, undo the process (backtrack) with the next number of backtrack item
- Looping the proses until it solved or unsolved (the puzzle is invalid)