Simple Colouring
Two-colour a digit's conjugate-pair chain; a colour that contradicts itself dies, and cells seeing both colours lose the digit.
How it works
- 1For one digit, link every house where it has exactly two homes left. Those links form chains.
- 2Colour each chain alternately. Along a chain the digit is either all one colour or all the other — nothing in between.
- 3If two cells of the SAME colour see each other, that colour is impossible: strike the digit from every cell of it.
- 4Otherwise, any uncoloured cell that sees both colours cannot hold the digit, since one of the two colours is true.