1. union division: find two lines of the same number in three parallel nine squares, and then use the nine squares rule to determine the position of the other line of numbers.

2. Patrol grid method: find out the number with high frequency in each nine-grid, and infer the position of this number in other nine-grids.

3. Exclusion method: observe the position where other figures in the ranking or nine squares can't be filled in, and fill in the remaining figures.

4. undetermined method: temporarily determine a number in a certain area, and then use it to exclude it.

5. Hypothesis method: fill in a number at random, deduce it, and deny the hypothesis if there is any contradiction.

6. Rank-and-column method: Starting from ranks, improve the efficiency of problem solving.

7. Frequency method: count all the situations in one line or nine squares, and select the number with high frequency in a certain position.