What are the criteria for determining the difficulty of Sudoku puzzles?
Is it a subjective estimation based on the author's experience, or are there any precise guidelines?
For example, there are some sudokus, where in each step there's always at least one digit that, if you look on some line, must be exactly in that position. Some, however, require to determine that field A can hold 2 values, but assuming it contains X, there's a contradiction in next step, so it must be Y... in other words, require either memorizing or making notices in the corner of the fields.
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