Sudoku

Thursday January 1, 2012

Puzzle No. XXX, grade

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HOW TO PLAY SUDOKU:

Simply fill the board such that each row, column and 3x3 box contains the numbers 1 to 9 uniquely.

To place a number, select the square on the grid you want to fill, then type the number you think belongs in there.
To remove the number, click the number again. You can also undo a series of moves by selecting the 'Undo' button, while selecting the 'Redo' button will allow you to redo your moves.

In 'Notes' mode you can enter more than one number in any square before progressively eliminating them as you unlock other parts of the puzzle.
To make notes, simply select your desired square and choose as many numbers as you think could be the possible solution.
Clicking the 'Undo' button will remove the numbers one at a time.

Selecting the Check button will reveal incorrect numbers in red - but the game leaves it to you to fix those cells.

The rules are very simple and you'll have no problem solving the easiest puzzles, but Sudoku is a very deep puzzle. Let’s have a look at the game in more depth.
The diagram on the right highlights one of the nine rows, in yellow. Every number in that row, from 1 to 9 must be placed, no duplicates.
One of the nine columns is shown in red. That must also contain the numbers 1 to 9. You can see that any number placed affects the contents of both a row and a column.
Last are the boxes, of which there are nine. The sixth box is shown in blue. Somehow, you have place 1 to 9 in each of those boxes as well!
Every Sudoku has clues. The minimum number is 17 but you'll normally find between 22 and 28 in this version of Sudoku. Strangely, the number of clues does not determine the difficulty of the puzzle.

In the puzzle to the right a cell is highlighted in GREEN. This is your first strategy: look for cells which have a lot of numbers in its row, column and box (marked in yellow). There is a good chance you will find that only one number remains. In this case, 6 is all that can go here. This eyeballing is essential at every stage. Now have a look at the cell highlighted in BLUE. It also has one remaining number. Can you work out which one?
Another pattern to look out for is placements based on the same number. You are looking for a cell in a box which can be the only cell left to place number X. Look at the centremost cell in this diagram. There are already two 2s in columns, marked with yellow squares. These remove the chance for a 2 everywhere else in the centre box.

These basic eyeballing strategies will only go so far before you are forced to consider more than one number for each unsolved square. This is when you can start placing 'notes, or small numbers on the grid.
You can place one to nine of the numbers on any cell that doesn't have a clue or solution. These notes are called 'candidates'. Arrangements of these candidates can give you clues to breaking a difficult bottleneck.
If you are interested in strategies for Sudoku do visit http://www.SudokuWiki.org.
Of course, you can always guess and see where it leads you. The undo and redo button will come in handy at this point.

Good luck!