Monday, October 20, 2008

Essential Sudoku DS: not impossible after all

I was all set this morning to unleash a torrent of wrath and indignation, complaining that puzzle 788 of Essential Sudoku DS was impossible. I had found a mere five squares which I could be sure about, and in a 15x15 grid that's pretty pathetic.

However, at lunchtime I tried a couple of deterministic paths - considering that this was a monocolour puzzle, I could test to see if certain spaces were on or off - and managed to complete half the puzzle. And then got stuck; I looked for twenty minutes and couldn't find a move.

This added impetus lead me to do a bit of searching - and I found a clever automatic solver which confirmed to me that there was a unique solution, and I'd got the half I was doing right. Then, after a full 5 seconds of looking at the puzzle again, I found the most obvious clue I'd missed (I had a block of five into which I had to fit a four, and I'd only coloured in a single square), and that led to me completing the puzzle.

So, in the hope that I can spare others of the anguish, here are some hints for puzzle 788 of Essential Sudoku DS:
  • On the bottom row, you're slightly limited as to where the 4 can go since there must be a maximum of 3 in any one place on the line above.
  • Test the top-right corner; see if setting that to 1 causes any contradictory issues.
  • The rows and columns with 5 and 4 together need only one square to be lost from the row or column in order for a single square to be coloured.
  • If all else fails, the picture is orientated diagonally from top-right to bottom-left.

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