Killer Sudoku

Unlike regular Sudoku, which provides pre-filled numbers to start, Killer Sudoku features a 9x9 grid with no pre-filled numbers. Instead, cells are grouped into 'cages' marked with a target sum, similar to the puzzle Kakuro. Solvers must use these arithmetic sums alongside standard Sudoku rules to place digits 1 through 9.

The 'Rule of 45' is a crucial solving technique in Killer Sudoku that relies on the mathematical fact that the digits 1 through 9 add up to 45. Because every row, column, and 3x3 nonet must contain each digit exactly once, their sum must equal exactly 45. Solvers use this rule to deduce the value of outlying cells within a cage.

To solve the cages, players must find combinations of unique digits (1-9) that add up to the target sum written in the corner of the cage. For example, a cage with a sum of 3 over two cells can only contain the digits 1 and 2. Combined with standard Sudoku logic, these arithmetic constraints slowly unlock the grid.

Many players find Killer Sudoku more challenging because it requires arithmetic and combinatorial deduction in addition to standard Sudoku logic. However, because all the information is provided upfront in the cages, it can sometimes feel more structured. The difficulty ultimately depends on the specific puzzle designer.