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Sudoku (or Su Doku):

There are just 3 rules to Sudoku:

1. Each row must have the numbers 1-9 occuring just once.
2. Each column must have the numbers 1-9 occuring just once.
3. And the numbers 1-9 must occur just once in each of the 9 sub-boxes of the grid.

Basic strategy

There are a number of ways to solve these puzzles. Here's one way: Basically, you go through each square and write down the list of possible values (given the constraints imposed by the columns and rows and 3x3 grid). You'll get something like this:

Then you go through and try to eliminate possibilities (trial and error, logic, etc.) until you've finished! Interested? There are four puzzles below, in increasing difficulty, for you to try out.

Oh, in case you're wondering, Sudoku is Japanese for "the digits must remain single"

Sudoku links:

• Here's a Sudoku Solver. When I get a sec, I'll code up my own solver, too.
• Here's how to make your own Sudoku. I've written up an algorithm for a simple Sudoku generator as well (see below)

Here's my pseudocode design for a Sudoku generator:

UPDATE: this code isn't very good. it will generate invalid boards that have the same numbers in a row or column. a better way would be to generate complete boards and remove numbers, the move numbers you remove the harder it will be

initialize 9 $smallBoards (3x3 grid)
initialize $largeBoard with those 9 $smallBoards

function ObtainCandidateValues ($largeBoard, $row, $column)
{
  get possible (candidate) values from a $smallBoard
  get candidate values from the $largeBoard row
  get candidate values from the $largeBoard column
  return $CVlist;
}

Main loop:

create a random list of reference to each of the 9 smallBoards
for each smallBoard in that list
{
  if $difficulty_level==hard, pick a number from 0 to 5, set it equal to the variable $places 
  if $difficulty_level==medium, pick a number from 2 to 4, set it equal to the variable $places
  if $difficulty_level==easy, pick a number from 3 to 5, set it equal to the variable $places
  create a list $places random locations (inside a $smallBoard)
  for each of those random locations
  {
     ObtainCandidateValues for that location
     randomly choose a value from the list and assign it to the location
  }
}									

Sample Sudoku puzzles