Sudoku

Hard sudoku

Hard sudoku expects you to work with pencil notes. Puzzles at this level can need subsets: pairs, triples and quads, where a group of cells shares a group of candidates and those numbers can be removed from everywhere else in the row, column or box.

When the singles run out, look for two cells in one unit that hold the same two candidates. The Hint button highlights the pattern on the board and explains the step.