Two ways to be sure
A sudoku is full when every row, every column and every box holds each symbol once. That one rule gives you two ways to be sure of a square.
The first is the easy one. Find a row, a column or a box with only one empty square. Whatever is missing goes there.
The second is the one people miss. Pick a symbol and look along one row. Cross off every square that cannot hold it. Its column or its box already has one. If a single square is left, that is where the symbol goes. That holds even when the square could still take other symbols.
So scan by symbol, not by square. Ask where the 4 can go in this row. That is faster than asking what one square can hold.
Work two squares
Start with the top row. It already holds 1, 2 and 3, so the last square has to be a 4. That is the easy step.
Now take the bottom row, which is empty, and ask where the 1 can go. The first column already has a 1. So does the second. So does the third. Only the last column is left, so the 1 goes in the bottom right square.
That square could still hold a 1, a 2 or a 3 on its own. Looking at the square alone would have told you nothing. Looking for the 1 along the row settled it.
Where does the 4 go?
The second row reads 3, empty, 1, empty. It still needs a 2 and a 4.
Look down the last column before you answer.
Try it
In the second row, where does the 4 go?
- The second column
- The last column
Show the answer
The second column
Correct. The last column already holds a 4 at the top. That leaves only the second column.
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