Killer Sudoku

Killer sudoku need not kill your brain!

Killer sudoku is one of the most popular of the sudoku variants. It is also perceived as one of the tougher variants, though through use of logic and practice, the majority of killer sudoku puzzles can be solved with methodology and not guesswork and panic!

Killer sudoku is most accurately described as a combination of kakuro and sudoku. It has the elements of sudoku in that each row, column and box must contain the numbers 1 - 9 once only in the standard shaped grid. However it has the element of kakuro in that the grid is split into different sized regions which sum to a specified number, and you have to work out the possible valid combinations to help place the numbers in the grid.

Unlike sudoku there are no given numbers at the start of the grid - everything can be deduced using logic. When they first see a killer sudoku puzzle many people are put off by the fact there are no givens and don't know where to start, but there are various strategies that can be used when trying to solve a killer sudoku puzzle.

You need to use pencilmarks frequently with killer sudoku. A good place to start is to find cells that have the least combinations and mark these in.

For instance, if there is a a set of two horizontal cells in a 3 x 3 box that sum to three, then you know it must be 1,2 or 2,1 so you can write these two options in those cells. You now know that, furthermore, the 1,2 in that row and 3 x 3 box cannot appear anywhere else, and therefore when doing the combinations for those cells you only have the other seven numbers to place and think about.

Through scanning the grid and marking such cells straight off you immediately have a good impact on the combinations and in fact may be able to place some cells.

Killer Sudoku Solving

Another rule you will need to use frequently is the 'only possible candidate' cell. Often you will have uncertainty where exactly numbers are placed in a region, but will know enough to place other numbers - learning to be vigilant for this sort of situation will really help you to solve killer sudoku puzzles.

Another tip for solving killer sudoku puzzles is to use what is known as the rule of 45. This is the fact that the numbers 1 to 9 sum to 45 and therefore the contents of each row,column and box must sum to 45.

You can often use this to solve a killer sudoku puzzle straight off. Imagine, for instance that you have a row whereby three cells are in one region, five in another, leaving just one cell at the end of the region that is part of a vertical region. By summing up the first two regions horizontal regions you know straight off the number to be placed in the final region - it is 45 minus the sum of those two regions, using the rule that the cells in that row must sum to 45 in total.

To play killer sudoku online, visit the Puzzle Club.

Killer Sudoku

Author: Dan

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Last Updated: Sep 10th 2006

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