Difficult sudoku puzzle
2005-11-29
Well, this sudoku puzzle isn’t really that special; just that it requires 2 ways of analysis to get one single digit right (highlighted in green circle). Most computer generated puzzles should be relatively straight forward, and just require a ‘linear’ way of analysis; but for this particlar ‘5′ it isn’t.
When analysed vertically, it is obvious the possibility 5’s can only be located on top right corner (5 in brown color) and bottom right corner (5 in green circle) of the 3×3 grid. When analysed horizontally, I marked the possibilities of 5’s in red square. The upper 2 pairs of red square can only be either 5 or 7, thus in the 3×3 grid the only choices of 5’s are located at bottom row. Bingo!
Of course there are lots of interesting sudoku puzzles as well, such as those providing only 17 digits yet still solvable (like this one); I have tried some, and surprisingly they are not particularly hard. To quote wikipedia:
Perhaps surprisingly, the number of givens has little or no bearing on a puzzle’s difficulty. A puzzle with a minimum number of givens may be very easy to solve, and a puzzle with more than the average number of givens can still be extremely difficult to solve.
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