Sudoku Solver- Tommy Montgomery

Find your favorite Sudoku board, or make up your own. Web Sudoku is a good place to find some.
Type in the static numbers.
If the board is valid, the square will turn green. If the board is invalid, squares with errors will turn red. A white background indicates an empty space.
After your initial board is filled in, check or uncheck the "Watch Recursion" checkbox.

Checking "Watch Recursion" enables you to watch the algorithm at work. It takes much, much longer to generate solutions to the board with this box checked (ranging anywhere from a few seconds to several hours,1 depending on the board).
Unchecking "Watch Recursion" generates the solutions very quickly, but you don't get to see the algorithm working (which isn't that enthralling anyway). It will take at most a few seconds to generate all solutions, unless the board is trivial or empty.

Press "Solve" to find the solution(s).
You can press "Stop" at any time to stop the solution-finding process, and look at the solutions the algorithm has found thus far.
Depending on the initial board, there may be more than one solution2 (or none). Press "Next Solution" to toggle through the different solutions (if there are more than 1000 unique solutions, then you only get to see the first 1000 the algorithm finds).
It is not recommended to press "Solve" with an empty or trivial board (i.e. not very many static numbers). It will take a long time, and there will be many, many solutions.3

Web Sudoku
Sudoku Solvers from Google
Another Sudoku Solver (the one I used to test my algorithm)

Despite the fact that there are seemingly hundreds of Sudoku solving sites out there, I created my algorithm independently from all of them (not that it's a big secret or anything). I used a basic recursive backtracking algorithm,4 which would account for the terrible efficiency.


Evil Puzzle #7,841,375,337	Evil Puzzle #7,841,375,337 solved

1My various tests with the applet show that if "Watch Recursion" is checked, Easy puzzles usually take under a minute to find all solutions, and Evil puzzles usually take a minimum of 4-5 minutes to generate all solutions. [back]

2Based on some extremely lazy and un-exhaustive research, "true" Sudokus have only one solution. However, it's more than possible to construct a valid board that will have more than one solution. [back]

3Interestingly, due to the cold, unfeeling, nature of the algorithm (which solves each board iteratively, testing each possible number sequentially [1, 2, 3, ..., 9]), it's possible to construct a seemingly trivial board that will take a long time to generate a solution. For example, this board:

takes quite some time to find the first solution. [back]

4I imagine that the algorithm used to solve "real" Sudokus (those with only one unique solution) are smarter, faster, and much more efficient than mine; see 3. [back]