Getting Started Getting StartedSudoku puzzles consist of a 9x9 cell grid, subdivided into 9 "boxes" consisting of 3x3 cells. Each row, column, and box is considered a "Group" of cells. Groups "Intersect" where they have some cells in common. Boxes share 3 cells with intersecting rows or columns, while row and columns intersect in just one cell.
Some of the cells are filled in with starting numbers. The object is to fill in all of the cells so that every group contains each of the digits 1 through 9. This is the same as saying that every group can contain each digit only once.
The general idea is to look for cells where only one of the digits is possible without violating the rule that each of the groups intersecting in that cell can only have one of each digit. The easiest approach is to start with a group that is mostly filled in already, then check each of its unsolved cells to see if intersecting groups rule out all digits except one. If so, fill in that cell with the solving digit. That new digit is now impossible in all three groups intersecting that cell, leading to new solutions.
Except for the easiest puzzles, you will encounter situations where no cells have only one possible digit. The more advanced strategies require you to consider what are all of the possible numbers for a cell, or what are all of the impossible numbers for a cell.
Moderate strategies generally find ways to reduce the possible numbers to only one, and you must list all of the possible digits in the cell before employing the strategy. As long as only a few possibilities remain, this is easy enough to do with pencil and paper. It gets messy when there are a lot of possibilities, so you should solve as much as you can with easier strategies first.
Advanced strategies require you to list all impossible numbers in all cells in intersecting groups, then look for special patterns. Filling in all cells with lists of impossible (or possible) numbers is tedious and requires good eyesight, a sharp pencil, and a sharp eraser. These strategies are usually employed as a last resort. Fortunately, Zudoku makes this much easier.