Basic Solving Sudoku Technique
OneChoice, or Sole Candidate, or Only Candidate, or Naked Single

OneChoice, or Sole Candidate, or Only Candidate, or Naked Single:
An empty square can only be filled with a number because all other numbers are taken in its row, column, or box.

Below is an example of using OneChoice(a.k.a. Sole Candidate, Naked Single) to fill R1C1
(the square in the first row and the first column).

For the empty square R1C1 (the square in the first row and the first column), number 5 is its only choice because

Numbers 1 and 2 show up in the first row where R1C1 is in.

Number 3 already shows up in the first column where R1C1 is in.

Number 4 already shows up in the first box where R1C1 is in.

Numbers 6, 7, 8 show up in the first box where R1C1 is in.

Number 9 shows up in the first row where R1C1 is in.

How to find them?
Scan the boxes (or rows, or columns) which has many numbers filled.
When you see such a box, check
every empty square in this box to see whether it has only one candidate left. If it is,
you can fill this empty square with this only candidate.


