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| Almost every Killer Sudoku puzzle of moderate difficulty requires this strategy at some point, and its so useful you should be looking out for it at every stage. Fortunately it only requires simple addition and spotting a certain type of pattern. Since the basis of the puzzle is Sudoku, each row, column and box will eventually have solutions which always add up to 45. If we can identify a group of cages which almost covers a unit (or more than one unit) but has one cage either sticking out (and Outie) or poking in (an innie) we can make some very useful deductions. You will see two examples in the first diagram. This Killer Sudoku is symetrical - which will often yield a double Innie and Outie. The two red rings surround boxes 3 and 7. Both boxes contain complete cages apart from one two cell cage. Lets add up the clues for complete cages in each box. We have: Box 3 = 22 + 14 + 6 = 42 Box 7 = 6 + 16 + 18 =40 We therefore know that Box 3 is missing 3 which can only go in C7 and Box 7 is missing 5 which must go in G3. Because we are dealing with simple 2-cell cages we get the other halves as well. |
 Innies and Outies Example 1 |
If we proceed a little further down the puzzle we get another useful Innie and Outie, this time made up of two columns. Since we are using two columns, the number we are interested in arriving at is 90. On the left hand side ringed in red, the cages are 9 + 12 + 9 + 15 + 14 + 6 + 16 = 81, so 9 has to come from the 3-cell cage totalling 18. 9 can be placed in J2. As the puzzle is symetrical a similar technique can be applied to the other side. |
 Innie and Outie Example 2 |
I'm grateful to Deelight in France for this example. This shows a group of four boxes and an Outie in C3. The total of four boxes is 180 and if we add up all the cages including the cage starting in C3 we get 181, so 1 must exist outside the four boxes. As C3 is the only outie cell that's where 1 must go. Although the solver cannot as yet complete this Killer, you can Load it here and see the example. |
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