“Done it: way too easy!” bellowed Comino, clicking her stopwatch and elbowing me awake at 3.30am. “Now go out right now and get me another book — the 24-hour store will have them.” British Su Doku addicts may be ruthlessly devoted followers, but here in my flat in Tokyo, it seems that The Times has created a monster.
The call from London seemed innocent enough: “Su Doku has really taken off here and our readers are after something that goes a bit further,” says Mike. “Can you scour all the puzzle books of Japan to find a next generation of Su Doku with some magic X-factor?” Within days, there were Su Doku books in every room in the house, every coat pocket and every handbag. We went through hundreds of puzzles — Jigsaw Su Doku, Consecutive Su Doku, Super-Mega Hard Su Doku, Diagonal Su Doku — before the chance purchase of a puzzle book in Yokohama. There, at the back of the slim volume, was our first sighting of Samunamupure — or “Killer Su Doku”, as it is known in the UK.
This was the one. After translating the rules aloud, Comino set about solving it as she stood there in the street. Within seconds, the glazed look of a true addict descended over her face, and a monster was born. I took the book from her hands, did the puzzle next to it, and a second monster followed immediately.
The great beauty of Killer is not that it is much more difficult than original Su Doku (although it can be harder to get started), but that it explores an extra level of logic and has greater subtleties. All the basic Su Doku rules are intact, but the guidance provided by the spaces enclosed by dotted lines (or sum boxes as they are known) creates a more varied challenge. The sight of a completely empty grid, with no starting numbers, may be daunting, but it gives the puzzler a real sense of cracking a code in a way that basic Su Doku does not.
As our addiction to Samunamupure took hold, we unlocked various ways of enhancing the experience. Essentially, you hop around the grid looking for places where the possibilities can be reduced to an either/or choice between just two numbers.
Certain elements of Killer actually make it more straightforward to solve than the harder Su Dokus. Since numbers cannot be repeated within a single sum box, if you see a sum box with three squares and the total is 24, its contents can only be 7, 8 and 9. Similarly, a sum box with three squares and a given total of 6 can comprise only 1, 2 and 3. A two-square sum box with a total of 17 must be 8 and 9, and because you cannot have two 8s together, a two-square sum box with a total of 16 can only be a 7 and 9. The other, most important, thing to remember is that the numbers 1 to 9, all added together, come to 45. Thus, since the normal rules of Su Doku still apply, every column, row and 3x3 bold-edged squares must have a total of 45.
How to do the numbers
In the puzzle there are several good places to start, but the most obvious is the top row. Across the top row there are four complete sum boxes and a stray square in the dead centre that forms part of another sum box. If you add up the intact sum boxes (7+17+7+9 = 40) then the missing number must be 5 because the row must have 45 as its total. Once you have put that 5 in place, the rest of the top row becomes clearer. The sum box totalling 17 must be the 8 and 9; we do not know the order yet, but it doesn't matter and should just be jotted down. The 5 is spoken for, so the two sum boxes totalling 7 must comprise 3-4 and 1-6 combinations because 5-2 is now impossible.
Though we still do not know what order they are in, the sum box totalling 9 at the right-hand end of the top row must comprise the 2 and 7, because they are the only numbers left unspoken for.
The next place to head for is the 3x3 bold-edged box in the bottom right corner of the grid. The sum box totalling 4 can contain only the 1 and 3 and the sum box totalling 16 can only be the 7 and 9 (again, we don’t know the order).
With those four numbers out of play, the sum box totalling 14 can only be the 6 and 8, leaving the 2, 4 and 5 still unaccounted for. Because the 5 is too big to appear in a three-square sum box totalling 7 (the one that spills over into the adjacent 3x3 box to the left) the 2 and 4 must be the ones to go there and you can write a 1 firmly into the spillover square of that sum box, leaving the 5 to be written in firmly as the lower half of the sum box totalling 12. This also means that you can put a 7 in the top half of that sum box.
With those numbers in place, the left half of the grid opens up at least three logical opportunities to enter more numbers, and the game is on.
Top tips from a couple of Killer Su Doku addicts
Think of right angles. If you have a two-square sum-box totalling 4, and you know it contains a 1-3 combination, look along the same row or column to another low-numbered sum box because you have now eliminated two numbers from the possible combination.
