# Classic. Latin square

Classic online Sudoku is a 9x9 square divided into smaller squares with a side of 3 squares (see picture). Initially, numbers are already set in some cells (from 1 to 9). The goal of the game is to fill in empty cells with numbers from 1 to 9. At the same time, numbers should not be repeated in each row, in each column and in each small square. A correctly formed puzzle has only one single solution. You can often come across the statement that the complexity of classical web Sudoku does not depend on the number of initially filled cells (keys). Sometimes this is true. However, this is often just a great excuse for many print media to publish puzzles with 30 or more keys, passing them off as Sudoku of various difficulty levels. By the way, the minimum possible number of clue digits in classic Sudoku is 17.

If you remove the small 3x3 squares from Sudoku, you get a slightly different kind of puzzle: the Latin square.

In our magazine, Sudoku of any size (starting from 5x5) are also classified as Classic / Latin square.

## Basic algorithms for solving Sudoku

Let's look at examples of two basic algorithms used to solve free online Sudoku:

1. In the highlighted vertical line (picture on the left) somewhere must necessarily be the number 9. Let's exclude the cells in which it cannot be: 3 central (because in the central square the number 9 is already present); The bottom 3 (the number 9 in the bottom square is also set) and a pair of top cells (they already contain the numbers 8 and 4). There is only one single cell left in the entire vertical, where the number 9 can stand. This algorithm can also be used for contour lines and small squares. The most populated blocks should be considered first of all. 2. Consider one of the puzzle cells (in the figure on the right). Let's exclude the numbers that cannot be in it: numbers 2, 3, 5, 9 (present in the small square to which the cell belongs); 1 and 8 (present on the horizontal) 4 and 6 (present on the vertical). It turns out that the only option for the selected cell is the number 7. These basic algorithms are sufficient for solving most classic Sudoku (and often difficult puzzles as well). But in the process of solving, using the marks in the cells, you can come up with many other algorithms, thereby speeding up the solution of puzzles.

Most importantly, take your time to solve Sudoku! After all, this is a relaxing game.

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