How to do a Sudoku

A Sudoku is a partially filled 9x9 square in which each of the nine rows, columns and 3x3 boxes are to be filled with the numbers 1 through 9 with no duplication of a number within them. In Japanese sudoku means something like "simple choice" or "single number". In order to complete the Sudoku one first tries to eliminate the single options by asking the questions "Where can I place this number?" in a row, column or box or "What number goes here?" referring to some unfilled cell. Basically it's "where?" or "what?".

It helps if you know a "bag of tricks" for finding the single choices. I call them tricks because they are ways in which you can end up being "fooled" and trick can also mean a skill. The first trick involves identifying the rows and columns passing through some box which contain the same number and checking to see where that number can be placed in the box. Only one cell may be possible since one cannot duplicate the number in the rows and columns and the box is already partially filled. The second trick is to look at the empty intersection of some row and some column and check the numbers that have already been used there to see if only one possibility remains for the empty cell. The third trick is a variant of the first but considers the contents of a box in addition to the contents of the rows and columns to eliminate the possibilities. This trick involves the greatest amount of "scanning" and is the one I seem to miss the most often. Maybe I need more practice. These are the basic tricks but they do not always work and one may have to add to one's bag of tricks to solve the Sudoku. An additional trick might be to look for rows or columns with two remaining unfilled cells and checking to see if some number can be restricted to either one of them which would allow one to eliminate the possibility from elsewhere in the row or column.

That's it. As Einstein said, "Keep it simple."