22. Patrol grid method: find out the number with high frequency in each nine grids and get the position of the number in other nine grids. This method is applied after method 1.
33. Exclusion method: This method is the key to solving problems and is easily overlooked by ordinary people. Observe each row or nine squares. If there is a place where no other figures can be filled in, fill in the remaining figures.
44. undetermined method: this method is not commonly used but very effective. Temporarily determine a number in a certain area, and then use it to exclude it.
55. Hypothesis method: that is, a number is randomly filled in a certain position, and then it is deduced, which may eventually lead to contradictions and negative conclusions.
66. Rank-and-column method: This method is used in the final stage to improve the efficiency of solving problems by breaking through ranks first.
77. Frequency method: Compared with the former method, this method can improve efficiency. List all the situations in a row or nine squares, and then choose the number with high frequency in a certain position.