## Sudoku

Today sudoku is, without doubt, the most popular logic puzzle. Although it seems simple, its solving requires numerous strategies. The first sudoku was made about thirty years ago.

## How to solve sudoku?

The basic rule of sudoku is illustrated in this example. Number six cannot appear in the cells marked with "х". |

In order to solve the puzzle, one must use only logic reasoning. Any randomly placed number, almost surely puts you back at the start. Here are some basic methods for solving sudoku:

Example 1:

This is the most typical example how to solve sudoku. As you can see, number 9 has only one free cell in the top right 3x3 square. |

Example 2:

In this example we can see that in the fourth row there are six numbers already written. The only missing numbers are 5, 7 and 8. Number 5 cannot appear more than once in the 3x3 square, so we conclude it must be in the coloured square. |

Example 3:

If we look at the second column, we can see that number 4 must be in the coloured square. |

Example 4:

In this example we can see that the only number that can be written in the coloured square is 8. If we place there any other number, it would appear more than once in a row, column, or 3x3 square. |

Example 5:

The so called "hidden pair" appears in harder sudoku puzzles quite often. Numbers 4 and 6 will surely be written in the coloured squares. Right now we cannot be sure what is their correct position, but it is important to know that in the coloured squares no other number can be placed. |

At the beginning, it is helpful to look at the rows, columns or 3x3 squares that already have lot of numbers written. That would usually give you a good start. Also it is useful to start with solving the number that appears many times.