Expert Sudoku

Expert puzzles need at least one advanced pattern, or give so few numbers that hidden pairs and triples have to carry you. Keep full notes; the patterns are hard to spot without them. Par is 25:00.

How to solve an expert Sudoku

• Hidden pair. Two numbers that can only go in the same two cells of a unit. Those cells can hold nothing else, so clear their other candidates.
• X-Wing. If a number can go in only two cells in each of two rows, and those cells line up in the same two columns, the number must sit on opposite corners of that rectangle. Remove it from every other cell in those two columns. Swap rows and columns for the mirror case.
• XY-Wing. A pivot cell with candidates X and Y sees one cell with X and Z and another with Y and Z. Whichever value the pivot takes, one of those two cells becomes Z. Any cell that sees both of them can’t be Z.

Playing here

Tap a cell, then a number. Turn on Notes to pencil in candidates; they clear themselves when you place a number nearby. You get three hints per game and unlimited undo. Your game saves after every move, so you can close the tab and come back to it.

Stuck? A hint corrects a wrong number or fills in a right one, and you get three per game. The technique guide covers everything these puzzles need.