Full House
Last Digit
1-step reasoningDifficulty
A row, column or box has only one empty cell left: fill in the missing digit.
Rule
A unit already holds 8 digits → its last empty cell takes the 9th.
Find an almost-full unit
Box 5 already has 8 digits; only one cell is empty: r5c5.
The idea
Every row, column and box must contain each digit from 1 to 9 exactly once. When a unit already holds eight digits, the missing one has only one place to go: the single empty cell.
It is the simplest technique of all, yet easy to miss while you are looking elsewhere. Each time you place a digit, glance at its row, column and box to see whether one of them has just reached 8 of 9.
How to spot it
- Count the empty cells in each row, column and box — any unit with exactly one is a target.
- Watch the units you have just added a digit to: they are the most likely to hit 8 of 9.
- Find the missing digit by counting 1 to 9, or by subtracting the sum of the present digits from 45.
Common mistakes
- Counting in a hurry gives the wrong missing digit — double-check with the sum of 45 if in doubt.
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Related techniques
References
The examples on this page are generated from random puzzles, not taken from these sources.