Sudoku - brain challenge

Sudoku (数独 sūdoku?, digit-single) is a logic-based,combinatorial number-placement puzzle. 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.
Features:
- each puzzle is generated randomly and have a unique solution
- The difficulty is from easy to very hard
- save game progress automatically and resume at any time
- tap a button to get the solution
- tap a button to get all the possible numbers for each cell
- with a pen tool to draw numbers like on a paper
- in custom mode,input your owner puzzle and get the solution easily
- unlimited undos