Killer Sudoku
Killer Sudoku is the next step for anyone who solves plain sudoku on
autopilot. The classic rules all hold: every row, column and 3×3 block must
contain the digits 1–9 without repeats. But there are almost no givens at the
start — instead the grid is split into cages, each carrying a sum in its
corner: the cage digits must add up to that number and not repeat inside the
cage.
The beauty of the genre lives in the rule of 45: every row sums to 45. When
cages almost cover a row, the last digit comes out by subtraction. A
two-cell cage summing to 3 is always 1 and 2; summing to 17 — 8 and 9. These
small deductions are what make killer sudoku feel more alive than the plain
version.
Every puzzle is generated on the fly: first a full solution is built, then
cells are removed — but only while the solution stays unique — and the cages
are carved over the remaining field. Grids are checked for consistency
automatically: a player can never meet an unsolvable variant.
Input is built for the phone: tap a cell, pick digits on the on-screen pad,
erase with one button. «Check» honestly highlights mistakes in red, «Hint»
places a correct digit. Three difficulties differ in the number of open
cells: «Easy» to learn, «Hard» is almost pure cages.