Sudoku Exercise

Do you like Sudoku? It will bring you a total fresh feeling. Sudoku is a game to develop intelligence, can exercise your logical ability and then improve it.
Basic Rules:
This Sudoku has 4 levels, just choose a proper one for yourself to start the amazing journery!
The objective is to fill a 9×9 grid with digits so that each column, each row, and each of the nine 3×3 sub-grids that compose the grid contains all of the digits from 1 to 9. No omission, no repeat. Only logical ability needed, regardless of math calculation. It is easy to play, a good way to excerice your brain due to the variety of the digital arrangement.