Solving Suduko

There are three main ways of solving the fiendishly difficult Sudoku.  In this part, you will learn about the scanning method.

When you start a Sudoku puzzle, you scan it, and then you continue to scan the puzzle periodically throughout the solution.  There are two basic techniques when it comes to scanning.

Firstly, there is the method known as Cross Hatching.  This is where you can the rows (or columns) to determine which line in a region contains a certain number, by a process of elimination.   This is then repeated with each column and row.  If you scan each number in order of their frequency then you can solve the puzzle quicker.  You must be systematic with this process, checking each number from 1 to 9.

The second method involves counting the numbers 1 to 9 in each region, rowsand column to identify the missing numbers.  Counting based on the last discovered numbers may well speed up the search.  In tougher puzzles the easiest way to determine the value of a single cell is by counting in reverse.  That means scanning the cell’s region, row and column for values that cell cannot be.  Through this, you see what numbers are left.

Once you are more experienced with Sudoku you may start to use what is known as “contingencies” whilst you are scanning.  This is where you narrow the possible location of a number to two or three cells within a region, row or column.  These possibilities can then be used to help you eliminate other cells as you continue with the cross hatching and counting.

If the puzzle is particularly fiendish, then you may have to use multiple contingencies which will mean keeping your pencil eraser handy as your brain does somersaults to keep them all in their right place.

If a puzzle can be solved solely by scanning, without having to identify contingencies, then it is classified as an “easy” puzzle.  More difficult puzzles are those that cannot be solved by this method alone and will need the other methods described in the next two parts!