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Should show which numbers I filled in and which were provided. Should let me mark multiple possible numbers for a square. Both are standard features of every other sudoku app.
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These don't have unique solutions. On my first puzzle I encountered a situation where there were 2 different solutions. This is a good start but I suggest you read up further on how to make sudoku puzzles.
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You should consider using a different color for the numbers that the player marks in, otherwise it's difficult to go back and see where you made a mistake.
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I'd like to suggest a 'how am I doing so far' button so folks can check their progress as they go. and also a timer to see how fast they are doing the puzzles, and maybe a pause button for timed mode. (http://www.websudoku.com/) Maybe with a timed mode, badges could be added for completing different levels in a certain amount of time.
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i only played the easy because i'm having an off day, but it is a good game, consider adding the ability to write notes in the boxes (possible numbers) and the hints need to be tweeked so that they don't go towards a box that has already been solved