Sudoku Puzzle Instructions:
Click on a cell and use your keyboard to type the numbers. Use the mouse or arrow keys to navigate. Erase with the space bar or backspace key. For mobile devices, touch a cell and press one of the input buttons. The blank button clears the cell.
There are three options:
NEW - Start a new game.
HINT - Give a hint about a square to try.
SHOW - Provide the solution for the puzzle.
Of the four levels, 1 is the easiest and 4 is the hardest.
A sudoku puzzle usually consists of a nine-by-nine grid. Some of the spaces contain numbers; the rest are blank. Your goal is to fill in the blanks with numbers from 1 to 9 so that each row, each column, and each of the nine three-by-three blocks making up the grid contains just one of each of the nine numbers. Sometimes sudoku is misspelled as sudogo, sudogu, or sudoko.
The minimum number of clues required for a Sudoku puzzle to have a unique solution is 17. The maximum number of clues a Sudoku puzzle can have without having a unique solution is 77. The number of all possible Sudoku puzzles is 6.67×1021.
 Gary McGuire, Bastian Tugemann, Gilles Civario, There is no 16-Clue Sudoku: Solving the Sudoku Minimum Number of Clues Problem, January 1, 2012, arXiv:1201.0749v1, http://arxiv.org/abs/1201.0749v1
Acknowledgement: This game is derived from Alf Magne Kalleland (www.dhtmlgoodies.com)