Sudoku

Solving a sudoku puzzle can be rather tricky, but the rules of the game are quite simple.
A sudoku puzzle is a grid of nine by nine squares or cells, that has been subdivided into nine subgrids or "regions" of three by three cells.

The objective of sudoku is to enter a digit from 1 through 9 in each cell, in such a way that:

• Each horizontal row contains each digit exactly once
• Each vertical column contains each digit exactly once
• Each subgrid or region contains each digit exactly once

This explains the name of the game; in Japanese, sudokumeans something like "numbers singly".
Solving a sudoku puzzle does not require knowledge of mathematics; simple logic suffices.
In each sudoku puzzle, several digits have already been entered (the "givens"); these may not be changed.
The puzzler's job is to fill the remainder of the grid with digits –respecting, of course, the three constraints mentioned earlier.
A "good" sudoku puzzle has only one solution.
In spite of the game's apparent simplicity, sudoku can behighly addictive... While the first sudoku puzzle was published as early as 1979 (back then, it was called "Number Place"), the game's popularity really took off in 2005; it can now be found in many newspapers and magazines around the world.