Tips and Tricks by Mark

[NickCarney94]

One of the most important tips is to try and establish where the 1s and 9s go in the grid. This should almost always be the first stage of the puzzle (ie position all the 1s and 9s and then go on to solve the remaining Sudoku puzzle). Paradoxically, the best way to establish the position of the 1s and 9s is often to work out where 1s and 9s can NOT go, and the app allows you to put an X in such a cell. We strongly recommend that you use this technique, especially for the harder puzzles. For instance if you know where the 1 is in a row, and the row's clue is 4, the 1 and 9 can only be separated by a single 4. So you will be able to tell that a 9 can only be in a maximum of two possible cells in the row. In this situation, place an "x" in all the other positions.

It helps to learn the combinations of numbers that can make up the sums: a row/column with a 35 total means that 1 and 9 must be at the extreme ends of a row or column, as 35 is the sum of the digits 2 to 8. A 5 could be a single digit 5 but it could also be a pair of digits 2 and 3. If you click on the clue outside the grid, the app will give you the range of possible constituents of the sum.

It often helps to work out what possible digits occur OUTSIDE the 1 and 9 in a row or column, by deducting the sum inside them from 35. So if a clue for a row is 33, there must only be a 2 "outside" the 1 and 9; moreover the 1 and 9 must be either in the second and last cell in the row, or the first and the penultimate cell.

Another more advanced tip is to note that if a row/column has a total of any number greater than 21 then its central cell cannot be a 1 or a 9. (This is because any number of 22 or higher requires a minimum of four cells as 8+7+6=21 and, if eg the 1 was in the central cell then the furthest away the 9 could be would only leave a three cell gap.)

If you are struggling with a particular puzzle, you can ask for a Hint. In the hints, there is some notation used to help you understand how to progress. For instance r1c2 means the cell in row 1 column 2; and r123c5 means the first three cells in column 5 (ie r1c5, r2c5 and r3c5). A "box" is one of the nine 3x3 boxes in the Sudoku and these are numbered in normal reading order. So box 1 is the 3x3 box in the top left corner of the grid; box 3 is the 3x3 box in the top right of the grid; all the way through to box 9 which would be the 3x3 box in the bottom right of the grid. The hints on some of the very difficult puzzles may occasionally refer to an X-wing (eg "r6r7c1c2 form an xwing"); this has a very specific meaning in our hints. We are saying that the 1/9 in question MUST be an the opposite corners of the four cells of this imaginary rectangle. (To put this another way, we're saying that the 1/9 in question cannot be in the SAME row/column within these four cells.)

If you are still finding progress difficult, especially on the harder puzzles then we recommend you visit the Cracking The Cryptic YouTube channel and search the channel for videos on "Sandwich Sudoku" or (this puzzles old name) "Between 1&9 Sudoku". You will find a lot of videos there explaining how to solve these puzzles efficiently.