This is a Sudoku problem, that is, the nine numbers in each small square can't be repeated, and the nine numbers in the vertical and horizontal rows of the big square can't be repeated. To do this problem, you can start from the second box in the first row, because there are the most numbers in it, just fill in one number, then fill in the series with numbers, and so on, first find the horizontal and vertical boxes with the most numbers, and then fill in.

Addition principle sum classification counting method

Addition principle: There are n ways to do something. Finish it. The first way has m 1 different ways, the second way has m2 different ways, …, and the nth way has mn different ways, so there are N=m 1+m2+m3+…+mn different ways to accomplish it.

The method of the first method belongs to the set A 1, the method of the second method belongs to the set A2, ..., And the method of the n method belongs to the set an, so the method to accomplish this belongs to the set A 1ua2u...uan.