# Solving Suduko

Suduko is a spatial and logic puzzle. Numbers 1 to 9 are scattered in 81 spaces with nine blocks of nine spaces marked by darker borders or shading. Letters or other symbols could be substituted for numbers without changing any of the concepts of the puzzle. The objective is to fill each block with the numbers 1 to 9. In doing so, each row and column of the puzzle must contain the numbers 1 to 9.

A word about difficulty: Suduko puzzle providers rank puzzles from easy to hard. The number of filled spaces, ranging from about 40 to 24, determines the difficulty—the fewer the filled spaces, the more difficult the puzzle. The position of numbers has some effect on difficulty. The label, however, may have no relationship to difficulty. I offer these two puzzles, both marked, "Moderate."

## Solving a Puzzle

Here is a puzzle with 27 filled spaces, a moderately difficult puzzle:

## Solution Techniques

There are three techniques that solve most suduko puzzles:

- Triangulation—using the position of each number to determine the position of the same number. This addresses the spatial aspects of the puzzle;
- Elimination—using several numbers to determine the position of one or more of them. This addresses the logical aspects of the puzzle; and
- Ghost numbers—using the possible position of numbers to determine other numbers. This preserves information you discover and is used in combination with triangulation and elimination.

## The Killer Error

A common mistake is to put a number in a row, column, or block that already contains the same number. It sounds like a stupid mistake. It is. It is also easy to make.