Sudoku originated in Nine palace map in China's Book of Changes, and sprouted in Switzerland at the end of 18. At the beginning of 19, it was named "Sudoku" by Japanese. Later, it quickly developed to European and American countries and carried forward. At present, this game has been popular all over the world.

The English name is Sudoku, where "su" stands for numbers, "doku" stands for single thought, and Sudoku stands for each number only once. Sudoku is very simple. Players need to logically infer the numbers in the blanks according to the rules of numbers and Sudoku that have appeared on the disk. Only unique numbers can be filled in.

From a mathematical point of view, Sudoku is actually a logical reasoning game. According to the known numbers, the player fills the numbers in the blanks by logical reasoning, so that the numbers in each row, column and bold grid are 1~9, which cannot be repeated. The picture below is a Sudoku problem. If parents try, they will find that filling in the blanks requires careful logical thinking and infinite patience.

(2) What are the classifications of Sudoku?

Standard Sudoku: The most common form of Sudoku, which consists of 9×9 squares, some of which have been filled with numbers. According to the known numbers, the player fills the numbers in the blanks by logical reasoning, so that the numbers in each row, column and bold grid are 1~9, which cannot be repeated.

Super Sudoku: It is more complicated than standard Sudoku and consists of 16× 16 squares. Some more complex super sudokus include letters A to F in addition to the numbers 1 to 9.

Diagonal Sudoku: On the basis of standard Sudoku, it is required that the numbers on each diagonal are not repeated.

Sawtooth Sudoku: The layout of the grid is serrated, that is, the numbers in each row or column cannot be repeated.

Inequality Sudoku: On the basis of standard Sudoku, the restrictive conditions of inequality are added. As shown in the figure below, the unequal relationship between two adjacent numbers needs to be considered, and the unequal symbol indicates the size relationship between the numbers in two adjacent cells.