For the 9x9 Sudoku puzzle, answers will be any number between 1 and 9.
So for example, a row filled in with the sequence (1, 3, 1, 4) is incorrect—because it excludes 2 and repeats 1. The sequence (1, 3, 2, 4), though, works for a single row.
So, a possible row (4, 3, 1, 2) could intersect with a possible column (1, 3, 2, 4).
So, if a box already contains three numbers (1, 3, 4), then the last square in the box has to be 2.
This way, when you find out another number fits into one of the two boxes, you’ll know that 4 has to solve for the remaining square. Not only will this help you solve the puzzle faster, but it’s likely to make the puzzle feel less frustrating, too.
For instance, go through every clue provided for the number 2. If you find any surefire answers, fill them in. Run through this process once or twice. When you’ve filled in a good number of squares, more obvious answers will start to crop up (like, for instance, the last remaining square in a box, column, or row).
