Hard Sudoku

Hard sudoku online

A hard puzzle here begins with around 28 numbers, about a third of the grid. Scanning for obvious squares will get you a handful of numbers and then stop. From there on you make progress by ruling candidates out, and the Notes button becomes your main tool. Each puzzle is generated when you press New game and is checked to have exactly one solution. For something gentler, see medium or easy.

Working with notes

Switch on Notes and tap numbers to pencil the candidates into a square; tap again to remove one. Placing a real number removes that candidate from its row, column and box for you. Many players fill in the candidates for every empty square once the easy numbers are done.

Techniques for hard puzzles

Hidden single: a candidate that appears in only one square of a row, column or box belongs there, however many other candidates that square has.
Naked pair: two squares in the same unit that both hold exactly the same two candidates. Those two numbers can be removed from every other square in the unit.
Pointing pair: when a number’s only places inside a box lie in one row or column, it can be removed from the rest of that line outside the box.
Box-line reduction: the reverse. If a number’s only places in a row lie inside one box, remove it from the other squares of that box.
X-wing: a number that has exactly two possible places in each of two rows, in the same two columns, can be removed from those columns elsewhere.

Advice

Keep your notes accurate. One missed candidate will send you down a false trail.
When you are stuck, check each unit for pairs before trying anything more exotic.
A red number means a mistake, not a dead end. Undo it and carry on.

Hard sudoku online

Working with notes

Techniques for hard puzzles

Advice

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