Hard Sudoku
Up for a challenge? Try Hard Sudoku! Test your skills and enjoy the satisfaction of overcoming a Hard Sudoku puzzle.
What's Hard Sudoku
Playing Hard Sudoku demands substantial thinking. But for many seasoned players, it can be the perfect mix of challenge and fun. It involves using classic solving techniques. If you're an experienced player who hasn't formally learned these techniques, don't worry – you likely use them instinctively, even if you don't know their names.
Solving techniques for Hard Sudoku
Last Digit
It's the simplest technique of all: find a block, a row, or a column with only one empty cell and fill it with the missing number.
Hidden Single
Choose a number and look for a block, a row, or a column with only one cell where this number can be placed. Look in blocks first – it takes less effort.
Naked Single
Look for a cell where there is only one number that can be placed in that cell.
Pointing Candidates
Consider each block. Find cells in that block where a certain number can be placed. If these cells are confined to a row or column, in that row or column, this number cannot appear outside that block.
This technique itself does not allow you to put a number into a cell. However, it helps to eliminate candidates from cells.
Claiming Candidates
Consider each row. Find cells in that row where a certain number can be placed. If these cells are confined to a block, in that block, this number cannot appear outside that row. The same works for columns.
This technique too does not allow you to put a number into a cell. It helps to eliminate candidates from cells.
Hidden Pair
Consider a block, a row, or a column. If there are two numbers that can be placed only in two cells and these cells are the same for both numbers, then no other numbers than these two may appear in those two cells.
This technique is the most difficult of all the techniques for solving Hard Sudoku. Use it only when nothing else works.
How to play Hard Sudoku
Use highlight function
Click on a board cell that already contains a number to see how the highlight function works. Use this function a lot. It'll make it easier for you to navigate the board and find the next move.
Look for simple moves first
Look for Last Digit, Hidden Single, and Naked Single first.
If that doesn't work, look for Pointing Candidates, Claiming Candidates, and Hidden Pair. Once you find a way to apply one of these techniques and eliminate some candidates from certain cells, look again for Hidden Single – maybe now it's possible to apply this technique.
Use hints
Use the hint function if you get stuck. Never guess. Click on the light-bulb button and learn how the next move was possible to spot. See hint details – it will help you to understand the hint better. Hints will teach you, and soon you'll notice that your solving skills have improved.
Pencil notes
You may choose to use pencil notes or not. Either way is fine – choose whatever feels more enjoyable for you. Please note that solving Hard Sudoku without pencil notes is possible but extremely challenging.
More tips
Read tips applicable to all levels of Sudoku, not just Hard.