How to Solve Sudoku Without Guessing (When You're Stuck)
A well-made sudoku never needs guessing: there is exactly one solution, and every number follows from the ones already on the board. When you feel stuck, it usually means you have not tried the next technique yet. Work through these four in order.
1. Hidden single: only one spot
Pick a box, row or column and a number that is missing from it. Look at every empty cell in that unit and cross out the cells where the number already appears in the same row, column or box. If only one cell is left, the number goes there. Scan box by box for each number from 1 to 9.
2. Naked single: only one number
Pick one empty cell and list the numbers already in its row, column and box. If eight of the nine numbers are used, the missing one is the answer.
3. Locked candidates: pointing and claiming
If, inside one box, a number can only go in cells that share a single row, then that number cannot appear anywhere else in that row. Cross it out of the rest of the row. The same works with columns, and in reverse (a row whose only spots for a number are inside one box).
4. Naked pair
If two cells in the same row, column or box can only hold the same two numbers, say 4 and 7, those two numbers belong to those two cells. Remove 4 and 7 from the notes of every other cell in that unit. This often reveals a new single.
A routine that works
- Scan for hidden singles, number by number.
- Pencil in candidates for the remaining cells.
- Look for naked singles in your notes.
- Apply locked candidates and naked pairs, then go back to step 1.
In Inkdoku every puzzle is built so this routine always works, and the Hint button names the technique and explains why the next number must be what it is.
FAQ
Is guessing ever necessary in sudoku?
Not in a well-made puzzle with a unique solution. Very hard puzzles may need advanced techniques, but never a coin flip.
Should I write notes for every cell?
Fill in hidden singles first, then pencil in candidates. Notes make naked singles and pairs much easier to see.
What if I still can't find the next move?
Use a hint that explains the step instead of revealing a random cell, so you learn the technique for next time.