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crunched
Joined: 05 Feb 2008
Posts: 168
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 Posted: Sun Apr 20, 2008 6:26 pm Post subject: April 20 2008 Hard |
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| Code: |
+----------------+---------------+----------------+
| 568 9 68 | 456 2 348 | 1356 7 1368 |
| 5678 4 1 | 567 35678 378 | 9 3568 2 |
| 25678 258 3 | 1 5678 9 | 56 568 4 |
+----------------+---------------+----------------+
| 3 28 28 | 467 67 1 | 467 9 5 |
| 1 6 4 | 8 9 5 | 37 2 37 |
| 9 7 5 | 2 36 34 | 1346 1368 1368 |
+----------------+---------------+----------------+
| 4 158 78 | 3 1578 6 | 2 15 9 |
| 2567 125 9 | 57 157 27 | 8 4 1367 |
| 25678 3 2678 | 9 4 278 | 1567 156 167 |
+----------------+---------------+----------------+
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Play this puzzle online at the Daily Sudoku site
I still marvel at how others can frequently solve these puzzles in less than 30 minutes (oftentimes less than 15 minutes!)
I spent quite awhile on this last night; then perhaps an hour this morning before it cracked.
I looked for triples, quads, and all kinds of things. I even looked to see if there were any wings.
Finally, I realized that the bottom row in Box 9 had 1s, and no other box in R9 had any 1s. Voila, this eliminated the 1 in the "15" cell in Box 9, making this cell 5. From there, things fell accordingly.
I get the impression that I need to be more systematic, checking row by row, Box by Box, etc. Checking for pairs, then triples or quads, then checking for box line reductions, etc. I tend to just jump from one idea to another in a random pattern. |
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Marty R.
Joined: 12 Feb 2006
Posts: 5770
Location: Rochester, NY, USA
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| Posted: Sun Apr 20, 2008 9:08 pm Post subject: |
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| Quote: | | I get the impression that I need to be more systematic, checking row by row, Box by Box, etc. |
Yes, that is a necessity.
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Finally, I realized that the bottom row in Box 9 had 1s, and no other box in R9 had any 1s. Voila, this eliminated the 1 in the "15" cell in Box 9, making this cell 5. |
What you have there is called "locked candidates" or "box-line interactions." Always look for these, they're present in most puzzles. The other situation that this technique covers is when all the possibilities for a number within a box are in one row or column, so that number can be eliminated from the row or column outside of the box. |
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kragzy
Joined: 01 May 2007
Posts: 112
Location: Australia
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| Posted: Mon Apr 21, 2008 2:57 am Post subject: |
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Hi crunched,
I am no expert but I do use a systematic approach (suits my OC brain!). It always seems to work, at least with the puzzles on this site. I also confess to using the Sweep button right from the start. Having done Sudokus for several years now, I no longer get any pleasure from grinding away looking for "simple" cells - I want to get to the fun of applying the higher techniques as soon as possible.
So, with pencil marks in full display, my approach is:
1. General housekeeping - solve any immediately apparent give-aways then go through row by row, column by column, box by box looking for pairs, triplets, quads and quints (weirdly, the quads and quints seem to jump out more quickly than the pairs and triplets).
2. Go through each box in turn, looking for "box-line interactions". These provide two types of eliminations as Marty described above.
3. Check for X-wings - for each digit (1 to 9) look at each row and if there are only two of the digit, look for another row with the same two instances. Then check the columns in the same way.
4. Check for XY-wings - for each pair ab, look for another pair ac or bc, and then look for bc or ac respectively, in any row, column or box shared by the original pair.
5. Check for XYZ-wings - in each box look for a triplet abc that has a pair in the same box ab, ac or bc. If the pair is ab then look for ac or bc in the same row or column as the triplet.
By that time, all puzzles on this site should be solved, except for one annoying thing! Sometimes an elimination from a higher technique will yield a simple "housekeeping" solution that I miss, with the result that I keep plowing through the higher techniques for no reason.
In fact that happened in today's VH (21 April). An XY-wing resulted in a row with only one 7 in it. I didn't see it until I had completed all my XY-wing and XYZ-wing checks, and then laboriously went back and re-checked for new X-wings. Bingo, there it was, just waiting for blind Kragzy to see it.
All part of the fun - which ever way you get there, it's great to have a puzzle crack open before your eyes.
Cheers. |
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crunched
Joined: 05 Feb 2008
Posts: 168
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| Posted: Mon Apr 21, 2008 2:57 am Post subject: |
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| Marty R. wrote: | | Quote: | | I get the impression that I need to be more systematic, checking row by row, Box by Box, etc. |
Yes, that is a necessity.
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Would you care to share what you look for first, then second, and third, etc? |
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Marty R.
Joined: 12 Feb 2006
Posts: 5770
Location: Rochester, NY, USA
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| Posted: Mon Apr 21, 2008 3:55 am Post subject: |
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| Quote: | | Would you care to share what you look for first, then second, and third, etc? |
You and kragzy posted at the same time, so he may have answered your questions. If you want to ask again, please do. |
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