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Messages posted by GreenLantern
 
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Actually, I was too hasty. r1c3<>6 only advances the puzzle to this point:



And here, the puzzle is finally solved by making another uniqueness deduction. This is a BUG+1 grid which allows us to immediately conclude r8c2=1, and the puzzle is reduced to singles.
I think one feature that would be really useful would be to allow the user to enter a puzzle into a blank grid and then allow them to solve it after first checking that there are no errors and that it's a valid sudoku. If the validating routine can also grade the sudoku and assign a difficulty level like Easy, Medium, Hard, or Extreme, that would be even better.
I realize this is an old post, but I just joined the forum. This is a basic pattern called Naked Pair.

The '4' and '6' in r46c5 form a naked pair and eliminate all 4's and 6's in Box 5 and column 5.
A Unique Rectangle at r23c35 allows us to conclude that either r2c3=6 or r3c3=6 and therefore, r1c3<>6, and the puzzle is solved.

Some solvers like to avoid using uniqueness assumptions in their solution, but I don't see the point in that since by definition, a valid sudoku must have a single unique solution. Why limit the number of solving techniques in your toolbox?
 
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