Hard Sudoku

About Hard Sudoku

Hard sudoku puzzles have fewer givens and can’t be finished with singles and pointing pairs alone. At some point you will need a hidden pair, or a naked or hidden triple. Notes help a lot here.

Sudoku is a logic puzzle on a 9×9 grid split into nine 3×3 boxes. Some cells start filled in; your job is to fill the rest so every row, every column and every box holds the digits 1 to 9 exactly once. There is no arithmetic and no guessing: every puzzle here has exactly one solution, and you can always reach it by logic.

Also called: number place, sudoku puzzles.

How to play Sudoku

1. Select a cell by tapping or clicking it, or move around with the arrow keys.
2. Enter a digit with the number pad or your keyboard (1–9). Enter the same digit again, or press Backspace, to clear it.
3. Turn on Notes (the pencil, or press N) to pencil small candidate digits into a cell. On a keyboard, Shift plus a digit adds a note without switching modes. Placing a digit removes it from the notes in its row, column and box.
4. Use Undo and Redo (Ctrl+Z and Ctrl+Y) as often as you like. Hint fills in one correct cell, choosing the one that logic says comes next, and tells you why.
5. If a digit repeats in a row, column or box, both copies turn red straight away, so you can see the clash. A digit that breaks no rule stays as it is, even if it isn’t the answer: finding that out is the puzzle.
6. Want to be told when a digit is wrong? Switch on Check answers under the number pad. Wrong digits then turn red and count as mistakes. It’s an assist, like a hint: the puzzle still counts as solved, but it can’t set a best time.
7. The puzzle is solved when every cell is filled and no digit repeats in any row, column or box. Your time, the level and your best time appear when you finish.

Rules

• Each row must contain the digits 1–9 once each.
• Each column must contain the digits 1–9 once each.
• Each of the nine 3×3 boxes must contain the digits 1–9 once each.
• The starting digits (the givens, shown in white) can’t be changed. Your digits are shown in blue.
• A digit that repeats in its row, column or box is always shown in red, together with the digit it clashes with. That only shows what’s already on the board, never the solution.
• Check answers is off at the start of every puzzle. Turned on, it marks any digit that doesn’t match the solution in red and counts each wrong entry as a mistake. Using it at any point makes the puzzle an assisted solve: it still counts as a win, but it can’t set a best time.

Sudoku tips and strategy

• Hidden single: pick a digit and a box (or row, or column), and check where that digit could go. If only one cell is left, it must go there. This finds most early moves.
• Naked single: look at one empty cell and cross off every digit already in its row, column and box. If only one digit is left, that’s the answer.
• Pointing pair: if, inside a box, a digit can only go in cells that share one row (or column), it can’t go anywhere else in that row outside the box. Box/line reduction is the reverse: if, in a row, a digit fits only inside one box, remove it from the rest of that box.
• Naked pair: two cells in a unit that can only be the same two digits, say 3 and 7, take those two digits. Remove 3 and 7 from every other cell in that unit. The same idea works for three cells and three digits (a naked triple).
• Hidden pair: if two digits can only go in the same two cells of a unit, those cells hold those digits, so you can erase every other candidate from them.
• X-Wing: if a digit can only go in the same two columns in each of two rows, then one of each pair must be used, so that digit can be removed from those two columns everywhere else. Expert puzzles also use Swordfish, XY-Wing and chains of two-candidate cells.
• Use notes sparingly at first: scan for singles across the whole grid, then pencil in candidates only when you get stuck.