My university (Keio university in Japan) set this question in 2013's entrance exam. The red boxes include numbers from 1 to 9.
What's the solution for this sudoku?
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6$\begingroup$ i would very much like to know which university this is. :P $\endgroup$– QuintecJun 4, 2017 at 18:31
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1$\begingroup$ @smci why 2x2? There is no 2x2 constraint $\endgroup$– KevinJun 5, 2017 at 2:44
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2$\begingroup$ Counter-intuitively, the red boxes actually make it easier rather than harder $\endgroup$– KevinJun 5, 2017 at 2:45
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2$\begingroup$ @thecoder16 It's Keio university in Japan. $\endgroup$– Takahiro WakiJun 5, 2017 at 11:00
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1$\begingroup$ @smci these are commonly known as windoku or hyper sudoku. $\endgroup$– paramesisJun 5, 2017 at 22:36
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1 Answer
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Final solution
Step-by-step explanation
- In the upper right red square, there's only one possible position for 7.
- In the lower right red square, there's only one possible position for 5.
- In the seventh row, there's only one possible position for 7.
- In the eighth row, there's only one possible position for 6.
- In the second row, there's only one possible position for 3.
- In the eighth column, there's only one possible position for 3.
- In the bottom middle 3x3 box, there's only one possible position for 3.
- In the seventh row, there's only one possible position for 5, then only one for 4, then only 3 is left.
- In the lower right red square, there's only one possible position for 2, then only 1 is left.
- Now only 1 is left in the bottom right 3x3 box.
- In the eighth row, there's only one possible position for 7, then only one for 8, then only 4 is left.
- Now only 2 is left in the bottom left 3x3 box.
- In the first column, there's only one possible position for 5, then only one for 1, then only 6 is left.
- In the top left 3x3 box, there's only one possible position for 7.
- In the lower left red square, there's only one possible position for 6, then only 4 is left.
- In the second column, there's only one possible position for 1, then only one for 2, then only 5 is left.
- In the middle left 3x3 box, there's only one possible position for 3, then only 6 is left.
- In the sixth row, there's only one possible position for 2, then only 8 is left.
- In the ninth column, there's only one possible position for 7, then only 8 is left.
- In the fifth row, there's only one possible position for 6.
- In the seventh column, there's only one possible position for 1, then only 8 is left.
- Now only 2 is left in the top right 3x3 box.
- In the upper left red square, there's only one possible position for 8, then only one for 5, then only 4 is left.
- Now only 4 is left in the third row.
- In the upper right red square, there's only one possible position for 2, then only one for 1, then only 3 is left.
- Now only 4 is left in the fourth row and in the eighth column.
- Now only 1 is left in the fifth row, and only 7 in the second row.
And from there it's easy to get the final solution.
answered Jun 4, 2017 at 18:45