11
$\begingroup$

My university set this question in 2013's entrance exam. Also the red boxes include from 1 to 9.

What's the solution for this sudoku?

|improve this question
$\endgroup$
  • 6
    $\begingroup$ i would very much like to know which university this is. :P $\endgroup$ – Quintec Jun 4 '17 at 18:31
  • 1
    $\begingroup$ @smci why 2x2? There is no 2x2 constraint $\endgroup$ – Kevin Jun 5 '17 at 2:44
  • 2
    $\begingroup$ Counter-intuitively, the red boxes actually make it easier rather than harder $\endgroup$ – Kevin Jun 5 '17 at 2:45
  • 2
    $\begingroup$ @thecoder16 It's Keio university in Japan. $\endgroup$ – Takahiro Waki Jun 5 '17 at 11:00
  • 1
    $\begingroup$ @smci these are commonly known as windoku or hyper sudoku. $\endgroup$ – paramesis Jun 5 '17 at 22:36
12
$\begingroup$

Final solution

Ta-da!

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.

first steps

  • 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.

next steps

  • 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.

filling in bottom and left

  • 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.

getting there ...

  • 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.

more and more

  • 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.

Now we're basically done

And from there it's easy to get the final solution.

|improve this answer
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.