Sudoku Clan › Hard Sudoku
Hard Sudoku
Hard puzzles start with fewer than 34 clues, and scanning alone will not finish them. At some point you have to work out the only digit a cell can hold. Try one here.
Play a hard Sudoku
| 5 | 8 | 1 | ||||||
| 3 | 4 | 8 | ||||||
| 8 | 5 | 1 | 7 | |||||
| 7 | 2 | |||||||
| 2 | 1 | 4 | 6 | 7 | 9 | |||
| 6 | 4 | |||||||
| 3 | 1 | 9 | 7 | |||||
| 4 | 2 | 3 | ||||||
| 1 | 5 | 6 |
Hard puzzle. Turn on JavaScript to play it here, or open the game.
What makes a Sudoku hard
In an Easy or Medium puzzle you can always find the next digit by asking where a digit goes. A Hard puzzle has at least one moment where that question has no answer anywhere on the board. To continue you turn it round and ask what a cell can hold. When its row, column and box already contain eight different digits between them, the ninth is the answer. That is a naked single, and every Hard puzzle in Sudoku Clan needs at least one.
Hard puzzles start with 28 to 33 clues. With so few on the board, there are often only two or three moves open at a time, and finding them takes patience. What a Hard puzzle never asks for is a technique that removes candidates. Every step places a digit.
How to solve a hard Sudoku
- Take the scanning moves first: boxes, then rows and columns.
- When you are stuck, look for cells where a busy row crosses a busy column inside a busy box.
- For such a cell, go through 1 to 9 and tick off every digit you can see in its row, its column or its box. If one digit is left, place it.
- Go back to scanning after every placement.
Notes make it easier
You can finish a Hard puzzle without pencil marks, but they turn the hardest step into an easy one. If you write the candidates in the cells of a crowded area, a naked single is simply a cell with one note in it. Keep notes tidy: whenever you place a digit, remove it from the notes in its row, column and box. Sudoku Clan can do that clean-up for you: turn on Auto-remove notes in the settings.
How long should it take?
The target time for Hard is 14 minutes.
After hard
Expert puzzles are the first that need you to remove candidates before you can place a digit, with pointing pairs and naked pairs. If you enjoy this kind of careful logic, try Duel, the turn-based mode where a digit found with a harder technique scores more points.
Questions
How do you solve hard Sudoku puzzles?
Scan boxes, rows and columns for the only place a digit can go. When that runs out, pick a cell in a crowded area and work out which digits are still possible for it. A cell with one possible digit is solved.
Do hard Sudoku puzzles require guessing?
No. A properly made puzzle always has a logical next step. If you feel forced to guess, there is a move you have not spotted yet. The Hint button in the game will name it and explain it.
What techniques do I need for hard Sudoku?
Hidden singles in boxes, rows and columns, and naked singles. Nothing that removes candidates.