Among the 22 787 puzzles in our database, some have very unusual features. Here are a few of them.
Puzzle solved using only the "Last Cell" technique
The Last Cell technique (also called Full House) consist in filling the last cell of a house with the only missing digit in that house. This technique is the simplest one, and is used in all puzzles to fill at least the last cell.
For example, solving puzzle 13930 ends with 16 Last Cell's in a row:
We wondered if there are puzzles which can be solved using only this technique, obviously starting from a large number of clues.
As we had no such puzzle in our database, we created a puzzle by hand with 20 missing digits (puzzle 14209, 61 clues):
Having posted this example on r/sudoku on Reddit, we got two suggestions for puzzles with 60 clues: 15248 and the symmetric 15249:
Is there any puzzle with a smaller number of clues which can be solved using only the Last Cell technique ?
Puzzles with nice patterns
The Enjoy Sudoku forum has a thread with Sudokus with an original rare shape. Here are some of them:
Puzzle 13760 has two circles in the corners:
Despite this unusual shape, its rating is only 1140. Puzzle 13820 however is much more difficult:
Puzzle 14188 (medium difficulty) has clues distributed among 4 small squares:
Puzzles 14191 and 14192 have a spiral shape, while 14193 has another nice pattern:
Puzzles with empty rows, columns or rectangles
Puzzle 13812 has 3 empty rows and 3 empty columns:
This is also the case of puzzle 14194 but with a different pattern:
Puzzle 14199 has a 4 x 7 empty rectangle:
This is beaten by puzzle 14200 which has a 5 x 6 empty rectangle and by puzzle 14202 which has a 4 x 8 empty rectangle:
Finally, puzzle 14204 has the digits distributed in two corners with 3 empty blocks:
Conclusion
These are only a few examples among more than 10,000 puzzles currently stored in our database. As our collection grows, we expect to discover even more remarkable layouts and rare properties. If you know an unusual Sudoku puzzle, let us know!