Anti-King Sudoku Explained in One Short Example
A small grid with a box border down the middle shows the anti-king rule. Two 7s touching diagonally across the border break it, and a 7 with a 2 in the same cells is fine.
Transcript
The rule
Anti-king sudoku adds one rule to the whole grid. Two cells that touch, even only at a corner, can't hold the same digit.
Cells that share an edge are already in the same row or column, and cells inside one box already differ. The rule adds something new only for diagonal neighbors on opposite sides of a box border.
Worth memorizing
It only matters across box borders.
Inside a box, digits never repeat anyway.
Diagonal touches across a border are the ones to check.
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