Odd-Even Sudoku
Odd-Even Sudoku is a variant that looks simpler than it plays. The base is
familiar: every row, column and 3×3 block holds the digits 1–9 without
repeats. But some cells are marked: a circle cell accepts only even digits
(2, 4, 6, 8), and a square cell only odd ones (1, 3, 5, 7, 9).
From that comes a simple but powerful trick: every row holds exactly four
evens and five odds. Count the circles and squares in a row — and half the
options fall away on their own. A circle in a block that already holds 2
and 4 leaves a choice between 6 and 8 — which is nearly a solution.
Every round is generated on the fly: first a full solution is built, then
some cells are marked strictly in agreement with it, and open digits are
removed so that the solution stays unique. A mark never contradicts the
grid — there is nothing to check, but it feels good anyway.
The marks are drawn right in the cells: a blue circle and an amber square
are visible at first glance. Input is tuned for phones: tap a cell, an
on-screen digit pad, erase with a single button. «Check» highlights
mistakes, «Hint» places a correct digit. Three difficulties: «Easy» — many
open digits and little marking, «Hard» — dense marking and a nearly bare
field.