The object of Sudoku is to fill in the 9 by 9 grid with the digits 1 through 9 so that each row, each column, and each 3 by 3 box (in bold lines) contains each of the number 1 through 9 only once.
To enter a puzzle (such as from a newspaper), click in the squares until the correct number appears. The left mouse button counts up, and the right button counts down. Numbers which can't legally be placed (because the number appears elsewhere in the row, column or box) are skipped. You can also type in the numbers.
To solve, press the solve button. The button lights up green if successful, red if no solution is possible, or orange if it is taking too long and gives up. If this happens, try entering some more numbers.
If you would like to try to solve it yourself and need a hint, press the hint button. It will fill in a number and light up the button yellow if there is any spot where only one number could go, or if there is any number that can only go in one spot in a row, column or box. If not, then nothing happens. Try guessing some numbers. The Guess button puts in random (but legal) numbers.
If there is no solution, then the Hint button turns red. Try removing some numbers.
Press Clear to start over.
This program is a Java applet. Here is the source code, relased under the GNU General Public License
Written by Matt Mahoney