Sudoku techniques Sudoku solving technique #1 Sole Candidate When all the other possibilities for the sudoku cell have been removed, and only one choice remains - then it must be the correct value. Sudoku solving technique #2 Unique Candidate If a cell is the only one in the row that can contain a certain number, then it must have that number. This is because every row of the sudoku must have each of the values 1-9. The same applies to columns and 3x3 blocks in the sudoku. Sudoku solving technique #3 Col/Row/Block interactions If you know that a number must occur in a certain row, then you can eliminate that number as a possibilty for other blocks in the same row. Eg, if you know that the number 4 must appear in the 1st row of block 1 (2nd and 3rd rows being full) then you know that the number 4 cannot occur in block 2 or 3 in the sudoku. This applies to columns as well. Sudoku solving technique #4 Naked subset if 2 squares in the same col/row/block have the same 2 candidates, then you can remove those numbers as candidates in the same col/row/block. This rule extends to cover 3 squares with the same 3 candidates, 4 etc.
