Cross-hatch scanning is generally all that is necessary for 'easy' puzzles. This process, referred to as cross-hatching, is repeated for each row and each column. 'hidden' by the presence of the other marks. The 5 in this cell is called a 'hidden single' because it can only be in this single location, and that fact is Since a number can only appear once in any given column or row and must appear exactly once inĪny given 3x3 block, the easiest place to start is to first checkįor cells that must hold a value because no other cell in a 3x3 block can hold that number.įor example, in this case the number 5 is excluded from all but one cell in the top center 3x3 block. But in that top middle block only one cell can hold a 5. In that cell the numbers 4, 5, 6, and 8 are all possible. The dots in the cell in row 3, column 5, indicate that

You should always start a Sudoku by finding all the hidden singles. Despite the name, hidden singles are far easier to find than naked singles.

There is only one possible cell for a candidate. There is only one possible candidate for a cell a hidden single arises when This situation can arise for one of two reasons. When a candidate k is possible in only a single cell ofĪ row, column, or block, then that cell must be k. Hypothesis and proof and a sort of depth.Īll of these techniques are based on identifying all the possible 'candidates' for a cell (indicated by marks)Īnd then eliminating them one by one until only one possibility remains in a given cell.Ĭross-Hatch Scanning (looking for singles) When all that fails, the Sudoku Assistant resorts to Almost-locked set analysis can be extended to grids, where itĪnd also to what I am calling almost-locked ranges. What I'm calling 3D Medusa analysis, includingĪnalysis. The Sudoku Assistant uses several techniques to solve a Sudoku puzzle:

Extreme sudoku linked