Sudoku is a mathematical game that originated in Switzerland in the18th century. It's a logic game with paper and pen. Players need to infer the numbers of all remaining spaces according to the known numbers on the 9×9 disk, and ensure that the numbers in each row, column and thick line palace (3*3) contain 1-9, which is not repeated.

Sudoku is nine palaces, and each palace is divided into nine squares. Give some known numbers and problem-solving conditions in this eighty-one box, and use logic and reasoning to fill in the numbers of 1-9 in other blanks. Make each number of 1-9 appear only once in each row, column and palace, so it is also called "Jiugongge".

In fact, Sudoku problems can be divided into bachelor level, master level, doctor level and expert level according to the difficulty level. And each level can be subdivided into introductory questions, advanced questions and advanced questions.

Expert level is commonly known as ashes. This is completely another realm of existence. They are different from the first three levels of people. The old bird-level Sudoku is out of hobby, and the ashes Sudoku is out of survival needs. Old bird people must go to the bookstore to buy books themselves, and the ashes don't have to pay for themselves; Old bird level people should ensure a certain amount of energy to cope with the next day's work and study, but don't worry about the ashes, because their occupation is to fill in Sudoku and set up Sudoku. Many newspapers, magazines and books are happy to adopt their Sudoku games.

It is said that in the whole process of solving Sudoku, the sitting posture of ashes has been maintained: one hand holds the pen to fill in the answers, and the other hand keeps rubbing cool oil, pouring coffee and breaking instant noodles. .....