Diagonal sudokus usually have lines drawn through the diagonals to remind you of this rule. Tougher diagonal sudoku puzzles have additional variants, such as only odd or even numbers appearing in the diagonals.
If the box already contains 2 and 4, the row has 3 and 7, the column has 5, and the diagonal has 9, then you know the cell must contain 1, 5, 6, or 9. As you keep doing sudoku puzzles, you’ll start recognizing where numbers can go without writing all the numbers into the cells. Make sure you write the numbers small enough so you can fit all the possible options inside the cell.
As you add a new number to the puzzle, check the row, column, and box for the same value. Since the number can’t be repeated in the same section, erase any other times you see the number.
Similarly, if the only possible locations for a number in a diagonal are in a 3 x 3 box, you can eliminate that number from other cells in the same box.
These cells are called “locked candidates” because they’re locked into specific cells even if you don’t know exactly which one the number goes into.
Cells with the same numbers in the same section are called “naked pairs. ” You can also find “hidden pairs” if the numbers still only appear in 2 cells in the same section, even if the cell has other possible numbers. If 1 and 3 are only in a cell that has 1, 3, and 5 and another with 1, 3, and 6, then those numbers must be written there.
The cells should form the corners of a rectangle if you drew lines between them. This method works because you know the number can’t be in the same column or row.
