By now, virtually everyone has heard of Sudoku puzzles. In the standard puzzle,a 9 × 9 partially filled array must be completed with digits 1 to 9 so that everyrow, every column, and each of nine disjoint 3 × 3 ‘boxes’ contains every digitexactly once. There are many ways to vary the puzzle, including different sizes(e.g. 4×4 mini-Sudoku or the 100×100 ‘Sudoku-zilla’), different boxes (6×6 withrectangular boxes, jigsaw Sudoku), puzzles with arithmetic constraints (Kakuro),and crossword-Sudoku hybrids using letters.