Killer sudoku

Killer sudoku (also killer su doku, sumdoku, sum doku, sumoku, addoku, or samunanpure サムナンプレ sum-num(ber) pla(ce)) is a puzzle that combines elements of sudoku and kakuro.

More often, puzzles are printed in black and white, with thin dotted lines used to outline the "cages" (see below for terminology).

The name stemmed from a Nipponised form of the English words "sum number place."

Traditionally, as with regular sudoku puzzles, the grid layout is symmetrical around a diagonal, horizontal or vertical axis, or a quarter or half turn about the centre.

This is a matter of aesthetics, though, rather than obligatory: many Japanese puzzle-makers will make small deviations from perfect symmetry for the sake of improving the puzzle.

By convention in Japan, killer sudoku cages do not include duplicate numbers.

However, when The Times first introduced the killer sudoku on 31 August 2005, the newspaper did not make this rule explicit.

Even though the vast majority of killer sudoku puzzles followed the rule anyway, English-speaking solvers were confused about appropriate solving strategies given the ambiguity.

This revised rule stuck and the world standard[citation needed] is no duplicates within cages.

Generally the problem is best tackled starting from the extreme sums—cages with the largest or the smallest sums.

In the early stages of the game, the most common way to begin filling in numbers is to look at such low-sum or high-sum cages that form a 'straight line'.

A further technique can be derived from the knowledge that the numbers in all houses (rows, columns and nonets) add up to 45.

This '45' technique can also be extended to calculate the innies or outies of N adjacent houses, as the difference between the cage-sums and N*45.