This Java applet illustrates the process of solving a Sudoku riddle. The applet requires Java 5.0 or higher. Click on the applet and enter the the numbers by moving the mouse over a field and pressing a key. Clear a number with space.
The program will display entries that are without alternative because of Sudoku's constraints in green. Click on these entries to commit them. Often, commiting an entry will allow new non-ambiguous entries to be found. If Sudoku's rules are violated, e.g. because two '1's are in the same row, the violations will be marked in red. A field that is still not commited, but for which no number at all is possible, is marked by a red star.
You can save up to five Soduko states, and restore them with the respective button. If you enable "Advanced strategy" a second strategy is used on top of the standard one. It is based on groups of fields instead of single fields. Only in rare cases however does this strategy actually supply additional useful constraints. If you enable the check box "Show possibilities", the numbers that are determined as being possible for a field by the solver are shown in blue. Only up to 3 possibilites are shown. Fields where still more possibilities are remain blank for reasons of space and conciseness.
Have fun!
Author: Torsten Nahm
The applet jar also contains the source. The class Sudoku is a well structured solver, which uses general principles and abstracts from the geometry of the grid. The GUI however is basically a quick hack. The documentation in the code is extremely terse, but it should still possible to understand it. The jar also runs directly as application.
I place the applet into the public domain.